First Definition—From their external Organization.
- Body—divided into Head—Trunk—Abdomen.
- Head.—Principal seat of the organs of sensation.
- Organs of sight. Immoveable eyes, simple or compound, varying in number.
- Organs of hearing uncertain, probably connected with the antennæ.
- Organ of taste. Ligula or palate within the mouth, accompanied by the organs of manducation—a pair of mandibles and maxillæ and an upper and lower lip, or their representatives.
- Organs of touch. Principally two jointed antennæ or their representatives, and four jointed feelers—two maxillary and two labial.
- Trunk. Principal seat of the organs of motion.
- Organs of walking, running, or jumping. Six or eight jointed thoracic legs, in pairs.
- Organs of flight. Four wings or their representatives, mostly with branching nervures containing air-vessels; found in the majority of the class.
- Organs (external) of respiration. A double set of lateral spiracles, some for expiration.
- Abdomen. Principal seat of the organs of generation.
- Organs of motion. In the Myriapods many pairs of acquired legs; in the Thysanura elastic ventral or caudal appendages.
- Organs of respiration. A double series of lateral spiracles for inspiration in the majority; in some only a single series, and in others only a single pair.
- Organs of generation those common to the Vertebrata, but retractile within the body, attended usually by various anal appendages, particularly a forceps in the males, and an ovipositor in the females.
- Head.—Principal seat of the organs of sensation.
Second Definition—From their internal Organization.
- Sensation.
- Nervous System. A small brain usually subbilobed, crowning a knotty double medullary chord; nerves proceeding from the brain and other ganglions to all parts of the body.
- Circulation.
- Heart replaced by a simple alternately contracting dorsal vessel or pseudocardia, without arteries or veins, but filled with a white cold sanies.
- Respiration.
- Lungs replaced by tracheæ, which receive the air from the spiracles, and distribute it by bronchiæ infinitely ramified.
- Digestion.
- Liver and biliary vessels in most replaced by from 2 to + 150 floating hepatic filaments opening into the space between the two skins of the intestinal canal below the pylorus.
- Generation.
- Internal organs. Males—Vasa deferentia, and vesiculæ seminales, and the other ordinary organs. Females—Ovary usually bipartite, with palmate lobes; genital organs single and mostly anal; one sexual union impregnates the female for her life.
- Development. In their passage to their adult state, after they have left the egg, insects undergo several simultaneous changes of their integument or successive moults, and the majority assume three distinct forms, with distinct organs, which appear as rudiments in their second state, and are completely developed in their last.
In defining the Arachnida I shall only mention those particulars in which they differ from Insectæ in their external anatomy.
Class—Arachnida.
- Body.
- Head and Trunk usually not separated by a suture.
- Eyes. Two to eight, not lateral.
- Mandibles cheliform or unguiculate, representing the interior pair of the antennæ of the Crustacea.
- Palpi pediform or cheliform.
- Trunk. Legs eight or their representatives: tibiæ mostly consisting of two joints.
- Abdomen with from two to eight spiracles.
- Head and Trunk usually not separated by a suture.
- Sensation.
- Nervous System. A small bilobed brain crowning a double, knotty, medullary chord; nerves proceeding from the brain and other ganglions to all parts of the body.
- Circulation.
- Heart unilocular, inaurite, with a system of circulation by arteries and veins; blood a cold white sanies.
- Respiration.
- Lungs replaced by internal gills receiving the air by spiracles.
- Digestion.
- Liver, consisting of conglomerate glands, and enveloping the intestines[49]; hepatic ducts.
- Generation.
- Genital organs double, ventral; more than one sexual union in the course of life.
The external characters in this class are the same almost in every respect as those which distinguish the Phalangidæ, the whole difference consisting almost in the systems of circulation, respiration, and digestion. Perhaps some future anatomist may discover in the tribe just mentioned, that there is a nearer agreement between them and the Arachnida in these systems than is at present suspected, which would prove them true Arachnida. I am inclined to think that Phrynus and Gonyleptes, &c. breathe by branchial spiracles; but having no opportunity of examining living specimens, I dare not speak with any confidence on the subject.