The Microscope. Its History, Construction, and Application 15th ed. / Being a familiar introduction to the use of the instrument, and the study of microscopical science

by Jabez Hogg

Crustacea.

The crustaceans comprise a large assemblage of Arthropods, presenting great diversity of structure. Some of the parasitic species have become so simplified in organisation that they appear to present no relationship with the higher members of the class, yet it is certain that all the species, whether terrestrial or aquatic, belong to the same stock, and may have had origin in the same fundamental plan of structure. Essentially, the body consists of a large number of segments, to each of which is attached a pair of two-branched appendages; the external branch is termed the exopodite and the internal the endopodite. Five segments at the front end of the body unite to form a head, the appendages of the first two being situated in front of the mouth, and performing the office of feelers or antennæ, while those of the remaining three segments are transformed into jaws, the first pair of jaws being the mandibles and the following two pairs the maxillæ. The rest of the appendages are variously modified and to some are attached respiratory organs in the form of gills. Crustaceans are broadly divided from Centipedes, Millipedes, Insects, &c., by the presence of two pairs instead of one pair of antennæ, and by the possession of branchial and not tubular (tracheal) respiratory organs. Arachnida and some other species are again widely separated. The majority of the young on leaving the egg are quite unlike the parent, and only acquires their definite form after undergoing a series of changes. The earliest stage, which has been called the Nauplius, already referred to in connection with the barnacle, is a minute body showing no trace of segmentation, and provided with a single eye, and three pairs of swimming appendages, which become the two pairs of antennæ and the mandibles of the adult. This stage is by no means of invariable occurrence, but is chiefly characteristic of the lowest members, the Entomostraca, and is rare in the higher, Malacostraca. The typical crustaceans are shrimps, crayfish, &c., so familiarly described by Huxley. The zoæa stage of the crab, a minute transparent creature, which undergoes several changes, swims about flapping its long jointed abdomen, like some of the Entomostraca, and the shrimp in particular. The larva of crayfish, the so-called glass-crab, is very peculiar and interesting. The sessile-eyed series, in which the compound eyes are never mounted on a movable stalk, and to which the Isopoda belong, exhibits great diversity of structure as well as of habits and habitat. Some live in fresh water, most are marine, while others live on land and take to a parasitic life.

Fig. 382.—Male Gnathia, enlarged.

This genus contains Gnathia, in which the male and female are so dissimilar, that they are frequently referred to as members of two families. In the adult male the mandibles are powerful and prominent, and the head is large, squared, and as wide as the thorax. In the female, on the contrary, the head is curiously small and triangular, without visible mandibles, and the thorax is much dilated. The creatures are about one-sixth of an inch long, and of a greyish colour, and the destruction they bring about is due to their habit of boring into timber below water mark. [Fig. 382] represents an enlarged view of the male Gnathia. These crustaceans are vegetarians, and feed on wood. Other members of the group, known as fish lice, are much larger in size, and chiefly infest the cetacea, and bear in addition two large eyes. By means of their powerful fore feet the Cymothordæ attach themselves to both marine and fresh-water fish, showing a preference for the inside of the mouth of their host.

Fig. 383.—1. Cypris; 2. Cyclops; 3. Branchipus grubei.

The bar-footed group Copepoda are free living, and the thorax bears four or five swimming feet; the abdomen is without appendages. The best known fresh-water form is Cyclops, the structure of which serves as a type of the order. The body is, as is well known to microscopists, broad in front and tapering behind, being thus, when viewed swimming, pear-shaped in outline. The dorsal elements of the head are fused to form a carapace, which bears a single eye, from which circumstance it derives its name. The eggs are carried by the female in a couple of ova-sacs attached to the last segment of the thorax, and so prolific are these creatures that a female will produce over four thousand million young. The young when hatched is an oval Nauplius, which after two or three moults acquires the adult state. In the family of the Apodidæ we have an equally well-known crustacean, the Branchipus. In the Branchipodidæ the body is also elongated, but there are no appendages to the abdomen, which consists of nine segments, while there are eleven pairs of thoracic appendages. The head shield is not developed backwards, and the large separated eyes are supported on distinct stalks. In the male the second antennæ are converted into claspers. These crustaceans swim upside down ([Fig. 383]).

Cladocera (Daphniadæ of Dr. Baird).—The water-flea (Daphnia pulex) may be taken as the best known example of the order. The body of this little active animal is narrowed in front, and at the posterior end, where the carapace is deeply notched, is the tip of the abdomen bearing the pair of rigid barbed setæ from which the genus takes its name. At the front of the head is a large compound eye and two pairs of branched plumed appendages, antennæ. The first pair of these are small and simple. The jaws consist of the mandibles and the first pair of maxillæ, the second pair of maxillæ being obsolete in the adult. The thorax comprises five segments, each bearing a pair of leaf-like swimming limbs. The abdomen consists of three segments, and is destitute of limbs. The males are usually smaller than the females, and much rarer, being rarely met with before the end of summer.

Eggs are laid both in summer and winter, and are passed into a brood-pouch, separating the upper surface of the thorax from the backward extension of the carapace. Here the summer eggs hatch, but the winter set are enclosed in a kind of capsule developed from the carapace. This capsule, termed the ephippium, is cast off with the next moult of the mother’s integument (a process necessary for the gradual growth of the crustacean), and falling to the bottom of the water, gives exit to the embryos, which hatch in its interior, and the young born from these “ephippial” eggs produce young, which in their turn become mothers. It appears, then, the winter eggs are enclosed in capsules of more than usual hardness to enable them to withstand any degree of cold that might otherwise prove fatal to the parent. Dr. Baird found, on examining ponds that had been again filled up by rain after remaining two months dry, numerous specimens of Daphnia and Cyclops quadricornis in all stages of growth.[80]

We learn also from his investigations that the Daphnia have many enemies. “The larva of the Corethra plumicornis, known to microscopical observers as the skeleton larva, is exceedingly rapacious of Daphnia. Pritchard says they are the choice food of a species of Nais; and Dr. Parnell states that the Lochleven trout owes its superior sweetness and richness of flavour to its food, which consists of small shell-fish and Entomostraca.” These crustaceans abound in fresh and salt water. Artemiæ are formed exclusively in salt water, in salt marshes, and in water highly charged with salt. Myriads of these Entomostraca are found in the salterns at Lymington, in the open tanks or reservoirs where the brine is deposited previous to boiling. A pint of the fluid contains about a quarter of a pound of salt, and this concentrated solution destroys most other marine animals. During the fine days in summer Artemiæ may be observed in immense numbers near the surface of the water, and, as they are frequently of a lively red colour, the water appears tinged with the same hue. The movements of this little animal are peculiar. It swims about on its back, and by means of its tail, its feet being at the same time in constant motion. They are both oviparous and ovoviviparous, according to the season of the year. At certain periods they only lay eggs, while during the hot summer months they produce their young alive. In about fifteen days the eggs are expelled in numbers varying from 50 to 150. As is the case with many of the Entomostraca, the young present a very different appearance from the adult animals; and they are so exactly like the young of Chirocephalus, that with difficulty are they distinguishable one from the other. The ova of other species are furnished with thick capsules, and imbedded in a dark opaque substance, presenting a minutely cellular appearance, and occupying the interspace between the body of the animal and the back of the shell; this is called the ephippium. The shell is often beautifully transparent, sometimes spotted with pigment; it consists of a substance known as chitine, impregnated with a variable amount of calcium carbonate, which produces a copious effervescence on the addition of a small quantity of a strong acid to the water in which the shell is immersed. When boiled, Artemiæ turn red as their congeners, lobsters. Their shells may be said to consist of two valves united at the back, resembling the bivalve shell of a mussel, or simply folded at the back to appear like a bivalve, but are really not so; or they may consist of a number of rings or segments. The body of Cypris presents a reticulated appearance, somewhat resembling cell structure. Entomostraca should be narcotised and prepared for examination under the microscope as directed by Mr. Rousselet at pages 345, 346.

Insects’ Eggs, etc.

Tuffen West, del. Edmund Evans.

Plate VI.