SYNOPSIS OF THE BRANCHES OF ANIMALS.

GROUP A. PROTOZOA.

Branch I.—Protozoa. Characteristics already defined.

GROUP B. METAZOA.

Branch II.—Porifera. Sponges are animals with their cells arranged in two well-defined layers, one of which is internal (endoderm) and the other external (ectoderm). There is a middle stratum of cells (mesoglœa) which does not attain to the definiteness of a mesoderm. Sponges do not possess a body cavity (cœlome); they possess, essentially, a bilateral symmetry. The body varies greatly in shape, and is traversed by canals having large openings (oscula) and numerous small openings (pores) on the surface. The ectoderm is continued through the pores into afferent canals; the endoderm cells line most of the internal (efferent) cavities, and are mostly flagellate; the mesoglœa contains a skeleton of flint, lime, or spongin. Reproduction may be by budding or sexual (either hermaphroditic or unisexual).

Branch III.—Cœlenterata. Animals possessing radial symmetry. There is no body cavity (cœlome), but there is a primitive digestive cavity (enteron). The body consists of two layers, an ectoderm and endoderm; between these two layers there is a mesoglœa of jelly-like consistency; in the simplest cases there are no cells in the mesoglœa, but secondarily endodermal cells may migrate into it. Stinging cells are generally present.

There are two divergent types of structure. The more primitive one is the sessile tubular hydroid, which may be compared to a gastrula furnished at one end with a crown of tentacles surrounding the opening of the enteron, and fixed at the opposite end. The less primitive, derived form, is the active jellyfish (medusoid) type. The hydroid type often constructs a calcareous skeleton known as coral. One life history may present both types (alternation of generations). Reproduction is sexual or by budding, in the latter case often resulting in the formation of colonies.

Branch IV.—Echinodermata. Animals, the larvæ of which possess bilateral symmetry, while the adults have radial symmetry. Even the adults exhibit to a varying extent a tendency to bilateral symmetry. Lime is always deposited in the mesodermic substance (mesenchyme). From the primitive digestive canals of the larvæ pouches grow out to form the usually spacious body cavity and the characteristic water vascular system. The latter and the nervous system exhibit generally a typical five-rayed arrangement. During development there is a distinction between mesoblast derived from gut pouches and mesenchyme produced from migrated amœboid cells. Complicated metamorphosis.

Branch V.—Vermes. The term “worms” includes a “heterogeneous mob,” a collection of classes whose relationships are poorly understood. But they are of great zoölogical interest, for, amid the diversity, there can be discerned affinities with Cœlenterata, Echinodermata, Arthropoda, Mollusca, and Vertebrata. They possess a well-defined mesoderm. They possess bilateral symmetry and have, as a rule, a head, tail, dorsal and ventral surfaces, and right and left sides. The lower worms are unsegmented. In the higher ones the digestive tract extends from the head to the end of the body; a dorsal vessel is usually present above the digestive tract; the nervous system consists of a supra-œsophageal ganglion (brain) and a simple or more commonly double, ventral, ganglionated cord; there is a body cavity (cœlome) lined with mesoderm; true jointed appendages are never present.

Branch VI.—Arthropoda. Animals with bilateral symmetry. Bodies segmented, but not uniformly so. Several or all of the segments bear paired jointed appendages variously modified. Chitinous exoskeleton. Digestive canal beneath the heart. Supra-œsophagel ganglion (“brain”) connected by a nerve ring round the gullet with a double chain of ventral ganglia. Cœlome small in the adult. Sexes almost always separate. Often some metamorphosis.

Branch VII.—Mollusca. Unsegmented animals, possessing, fundamentally, a bilateral symmetry. A very characteristic structure is the “foot”—a muscular protrusion of the ventral surface. Typically, a projecting fold from the dorsal surface of the body forms a mantle, or pallium; the mantle often secretes a single or double shell covering the viscera. The mantle and shell may both be absent. The central nervous system consists of paired ganglia with connecting commissures, viz.: cerebral, pleural, pedal, and visceral ganglia. Heart possesses two auricles and one ventricle. Respiration generally by gills. Frequently there are two larval stages. Development is often direct.

Branch VIII.—Vertebrata or Chordata. Vertebrates are animals having a distinct body cavity (cœlome) and a segmental arrangement of parts. A hypoblastic skeletal notochord is always present in the embryo, but tends to be replaced by an axial skeleton of mesoblastic origin (backbone). Gill slits are always present in the embryo and may or may not persist in adult life. Gill-lamellæ do not occur above Amphibians. Heart is ventral. Central nervous system is dorsal. Eye begins to develop as an outgrowth from the brain.

Class 1.—Tunicata. The tunicates are remarkable animals, and seem to stumble on the border line between Invertebrates and Vertebrates. Their vertebrate characteristics—gill slits, notochord, dorsal nervous system, and brain eye—are generally discernible only in the free-swimming larval state. They generally degenerate as they progress towards the adult condition, and diverge greatly from the vertebrate type. They are mostly stationary. They are multicellular animals possessing bilateral symmetry. The body is enveloped by a thickened cuticle containing cellulose. The pharynx, perforated with the gill slits, is surrounded by a peribranchial chamber (atrium) which communicates with the exterior by an atrial opening. Heart is simple and tubular. The nervous system is generally reduced to a single ganglion. Nephridia are absent. Hermaphrodites. Usually a metamorphosis.

Class 2.—Leptocardii. Simple, worm-like vertebrate animals represented by a single distinct type—amphioxus or lancelet. The central nervous system consists of a spinal cord, and a very ill-defined, rudimentary brain. No skeleton other than an unsegmented and persistent notochord which projects beyond the anterior end of the nerve cord. No scales. Gill slits are very numerous in the adult. Amphioxus is widely removed from the fishes by the absence of skull, jaws, definite brain, sympathetic nervous system, ear, eye, genital ducts, spleen, and definite heart, the latter being simple and tubular. No pectoral or pelvic limbs (fins). Blood colorless; gastrula ciliated and free-swimming. Metamorphosis.

Class 3.—Marsipobranchii. Worm-like vertebrates having round mouths without distinctly developed jaws; no jaw-bones. Without scales and without paired limbs (fins). Six or seven gill pouches. Skeleton consists of persistent, unconstricted, cartilaginous notochord. No sympathetic nervous system. Single nasal sac. No conus arteriosus, no spleen or pancreas. Undivided segmental duct.

Class 4.—Pisces.[26] Aquatic, cold-blooded vertebrates, with a movable lower jaw. Cartilaginous or osseous skeleton, with paired pectoral and pelvic fins, supported by fin rays (radials diverging from several basal pieces); also unpaired fins. No sternum. Exoskeleton of bony plates or scales. Skull has usually one occipital condyle. Breathing by permanent gills attached to gristly or bony arches on the sides of the gill clefts. In most fishes the digestive cavity gives origin to an air bladder which may or may not remain permanently connected with the digestive cavity. It mostly serves a hydrostatic purpose, but in some of the more primitive fishes (Amia, Lepidosteus) it is occasionally of some slight use in respiration. Heart consists of single auricle and single ventricle, and contains only venous blood; there is a sinus venosus and often a conus arteriosus. No inferior vena cava. No allantois. Most fishes lay eggs which are fertilized in the water.

Class 5.—Amphibia. In the evolution of vertebrates the Amphibians represent those forms which made the transition from aquatic to terrestrial life, but have lingered near the water. Certain acquisitions gained by the Dipnoi, such as a three-chambered heart and lungs, have been accentuated and firmly established by the Amphibians.

Amphibia are cold-blooded vertebrates whose larval forms always have gill arches bearing gills. In some forms the gills are retained throughout life, though the adults always possess functional lungs. Nasal sacs open posteriorly into the mouth. In existing forms there is rarely any exoskeleton. There are two occipital condyles. Unpaired fins are frequently present, though they possess no fin rays. When limbs are present they possess distinct digits and conform to the same type as those of the higher vertebrates. Heart has two auricles and one ventricle. There is an inferior vena cava. The intestine ends in a cloacal chamber, as do also the urinogenital ducts. Eggs almost always laid in the water. Often a marked metamorphosis.

Class 6.—Reptilia. Cold-blooded, air-breathing vertebrates. Limbs usually ending in claws; limbs sometimes absent; exoskeleton of scales; ribs well developed; incomplete double circulation; heart is four-chambered in the highest forms; oviparous; no metamorphosis; embryo with an amnion and allantois.

The following are some of the orders that have become extinct, viz.:

The following are the living orders, viz.: a, Ophidia; b, Lacertilia; c, Chelonia; d, Crocodilia.

Class 7.—Aves. Feathered vertebrata; heart four-chambered; warm-blooded; lungs with accessory air-sacs; bones dense and hollow; jaws encased in horny beaks in modern forms; the fore-limbs form wings; oviparous; eggs are very large and covered by calcareous shell.

Class 8.—Mammalia. Animals having the body covered with hair; warm-blooded; young nourished with milk secreted from mammary glands; lower jaw articulating directly with the skull, the quadrate becoming the malleus (ear-bone); heart four-chambered and with the aorta reflected over the left bronchus; red blood-corpuscles non-nucleated; complete diaphragm; brain large, especially the cerebrum and cerebellum; uterine gestation; viviparous.

Primates are an order of Monodelphia or Placentalia, including lemurs, monkeys, and man. They are nearly all adapted to an arboreal life. The hand (manus) and the foot (pes) nearly always have five digits provided with flat nails. The pectoral and pelvic limbs are prehensile, owing to the fact that the thumb (pollex) and big toe (hallux) are more or less completely opposable to the other digits. The orbital fossa is completely surrounded by a bony rim. The femur does not have a third trochanter. The internal condyle of the humerus does not have a foramen above it. The clavicles are always well developed. The testes descend into a scrotum. On the thoracic region there are nearly always two teats. The placenta may be non-deciduate, or deciduate and metadiscoidal.

The order of Primates is subdivided into the following sub-orders, viz.:

Lemuroidea. The Lemurs are small, monkey-like quadrupeds. They are mostly nocturnal, arboreal creatures of comparatively low organization. The body is furry. The orbital and temporal fossæ freely communicate. The lachrymal foramen is situated outside the orbital fossa. The dentition of the Lemurs varies greatly; in some of them it is 2/2 1/1 3/3 3/3 for both jaws. In nearly all of them the median incisors of the upper jaw are separated by a median space. Both feet have flat nails on all the digits, except the second of the hind-foot, which has a claw. The thumb (pollex) and the big toe (hallux) are well developed. The cerebral hemispheres are only slightly convoluted, and do not entirely overlap the cerebellum. The transverse colon is almost always folded on itself. There may be a pair of teats on the abdomen. The uterus is two-horned, and the placenta is diffuse.

This sub-order comprises the Lemurs proper (Tarsius, Lemur, and other genera) and the Aye-Ayes (Chiromys).

Anthropoidea. The Anthropoidea are the most highly organized Primates. They are chiefly modified for an arboreal life. The body is hairy instead of furry, but only slightly so in man. The incisors do not exceed 2/2; the molars are 3/3, except in the marmosets, where they are 2/2. The upper, median incisors are in close contact. The orbital and temporal fossæ are separated by a broad vertical plate. The lachrymal foramen is situated inside the orbital fossa. All of the digits are provided with flat nails, except in the Hapalidæ, in which all except the big toe (hallux) are provided with a claw. In some the thumb (pollex) is rudimentary or absent. The clavicle is well developed. The cerebrum is usually richly convoluted, and more or less completely covers the cerebellum. The uterus has no horns. The placenta is deciduate and metadiscoidal.

The Anthropoidea are subdivided into the two following groups:

The Platyrrhinæ or New-World Monkeys are subdivided into the following families, viz.:

The Catarrhinæ or Old-World Monkeys are subdivided into the following families, viz.:

Hapalidæ. The Hapalidæ or Marmosets are no larger than squirrels, being the smallest monkeys. They are found mostly in Brazil.

There is a broad septum between the nostrils. Their dentition is 2/2 1/1 3/3 2/2, and is distinctive, for the remaining Anthropoidea have 3/3 molars. The tail is long, hairy, and non-prehensile. The arms are not longer than the legs. The thumb (pollex) is long, but is not opposable. All the digits except the great toe (hallux), which is small, have curved, pointed claws. There is no bony external auditory meatus. There are no cheek pouches, or ischial callosities. The Hapalidæ often bear three young ones at a birth, whereas the other Anthropoidea commonly bear but one. There are two genera of Marmosets, viz.: Midas and Hapale.

Cebidæ. The Cebidæ are most at home in Brazil. They have a broad septum between the nostrils. Many of them have prehensile tails. The dental formula is 2/2 1/1 3/3 3/3, and is characteristic. There is no bony external auditory meatus. There are no cheek pouches, or ischial callosities. All the digits are provided with flat nails. The thumb (pollex) is not opposable.

This family includes the following genera, viz.: Tee Tees (Callithrix), Howling Monkeys (Mycetes), Spider Monkeys (Ateles), Squirrel Monkeys (Chrysothrix), and Capuchin Monkeys (Cebus).

Cercopithecidæ. This family includes the Old-World dog-like Apes. They are quadrupeds, and the muzzle or snout is quite dog-like. There is a narrow septum between the nostrils. The dental formula is 2/2 1/1 2/2 3/3. The sternum is narrow. The tail is not prehensile. The cæcum has no appendix vermiformis. There is a bony external auditory meatus. There may or may not be cheek pouches. Over the rough surface of the everted ischia the skin forms callosities (ischial callosities) which are often brightly colored. The thumb (pollex), when present, is opposable. This family includes two genera, Macaques (Macacus) and Baboons (Cynocephalus).

Simiidæ. This family includes the Anthropoid Apes of the Old World. They are less quadrupeds than the former, often walking in a semi-erect position. The dental formula is 2/2 1/1 2/2 3/3. The sternum is broad. The cæcum has an appendix vermiformis. The nasal septum is narrow. The pectoral limbs are much longer than the pelvic ones. There is a bony external auditory meatus. There are no cheek pouches. Only in the Gibbons are there ischial callosities, in whom they are small. The thumb (pollex) is opposable. No visible tail. This family includes the following genera, viz.: Chimpanzees (Anthropopithecus), Gorillas (Gorilla), Orangs (Simia), and the Gibbons (Hylobates).

Hominidæ. This family of Anthropoidea includes only the human species (Homo sapiens). The dental formula is 2/2 1/1 2/2 3/3. The tail is not visible. The Hominidæ differ from the Simiidæ, structurally, mainly in the more perfect assumption of the erect attitude. There is a more complete adaptation of the pelvic limbs to bearing the weight of the body, correlated with compensating alteration in the curvature of the spinal column. The big toe (hallux) is not opposable, and also it is often longer, never shorter than the other toes, and is not abducted from them. He has a far better heel than the Simiidæ. The thumb is far more opposable in man. There is a greater length of the pelvic limbs compared with the pectoral limbs. The canine teeth are much smaller and do not project beyond the level of the others. The brain has a much greater size and complexity. Man has a larger forehead, smaller cheek bones, smaller supraorbital ridges, a less protrusive face, and a true, well-marked chin.

The Simiidæ are subdivided into the two following groups, viz.: