I. The Cirripeds, (cirripedia, cirripeda, cir rhopoda) ; oceanic animals called barnacles and acorn shells : they are characterized by their fixed condition, being either sessile, or attached to foreign bodies by means of a peduncle ; their generation is, consequently, hermaphrodite, without the intercourse of .se parate individuals, but the male and female organs are distinctly developed in each animal. The blood is colourless and is propelled by a dorsal vasiform heart, but the venous system is diffused. The branchke are internal. The cir ripeds undergo metamorphoses, but are ulti mately inclosed in an inarticulate defensive covering of shelly pieces varying in number, form, and size.
II. The Annelidans, ( Annelida, red-blooded worms,) are always locomotive ; and, conse quently, although hermaphrodites, they enjoy the intercourse of the sexes, and reciprocally fecun date each other. Their blood, which is gene rally red, like that of the vertebrate animals, circulates in a closed system of arteries and veins, which sometimes has appended to it several well-marked propulsive cavities or hearts ; they respire by means of organs some times developed externally, sometimes remain ing on the surface of the integument, or lodged in its interior. Their body, which is of an elongated form, and covered with a soft skin, is always divided into numerous transverse segments, of which the first, called the head, scarcely differs from the others, except by the presence of the mouth and of the principal organs of the senses. Many possess branchioe, arranged the whole length of the body, or situ ated at the middle; others, which for the most part inhabit tubes, have the branchim collected at the anterior part of the body ; in others, again, the respiratory organs are in the form of internal air sacs. The annelidans never possess articulated limbs, but many have, instead thereof, stiff bristles, or hooks, frequently inclosed in tubular prolongations of the integument.
The other articulate classes, viz., insects, arachnidans, and crustaceans, differ from the preceding classes in the possession of arti culated limbs, terminated by claws ; and in connexion with the superior powers of loco motion afforded by these appendages, the sexes are separate, and the organs of vision are well developed, and often highly complicated. With the exception of some genera, as the myriapoda, in which the body is divided into a number of nearly equal segments, and of the arachnida and many crustacca, in which the head and thorax are blended together, the body of the condylopes of Latreille is divided into three principal parts, viz., the head, which
bears the antenna, the eyes, and the mouth; the thorax, which supports the feet and the wings, when the latter are present ; and the abdomen, which contains the principal viscera.
These segments present different degrees of hardness in the different classes of condylopes, being most flexible in the arachnidans, firmer in the insects, and calcareous in most of the crustaceans. The origin of the insertions or articulations of the body which form so marked an external character of these animals, is as follows : The integument is composed of two layers or pellicles, viz., the epidermis and the corium, and is originally of equable consistence, and presents an uninterrupted continuity, save by Some slight transverse superficial wrinkles. The epidermis subsequently becomes solidi fied, in arachnidans and insects, by the super addition of a peculiar substance termed chitine, and in crustaceans by a calcareous deposition, so as to be divided into bands or rings. As the external development proceeds, these epidermic pieces are detached posteriorly from the inferior pellicle, or corium ; and the intervals of the segments remaining membranous, and preserv ing their flexibility, yield readily to the various movements and inflections of the body.
The I l Id class of articulate animals or In sects ( Insecta), are either myriapod or hexapod. Most of the latter are furnished with wings, which they acquire at a certain age, after undergoing metamorphoses varying in kind and degree. In every state they respire by trachea, or elastic vessels which receive the air by stigmata, situ ated along the sides of the body. A dorsal vessel propels the circulating fluid, which is afterwards diffused throughout the cellular tissue of the body. They have conglomerate or compound eyes, and antenna.
I V. The Arachnidans (Arachnida, Spiders, Scorpions, &c.), are octopod and apterous; they have no antennae, and have simple eyes. Their circulation is effected by a dorsal vasi form heart which transmits arterial branches, and receives the returning blood from veins. Their organs of respiration vary, some pos sessing true pulmonary sacs which open upon the sides of the abdomen, others receiving the air by trachea, like insects. In both cases, however, the air is respired by lateral orifices or true stigmata.