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Show 74 ON CLASSIFICATION. b n1 a r rami·, t}1e presence of 1nammary glands, and the nou-nucleated red blood-corpuscles appear to separat Mammals as widely from Birds and Reptiles as the latter are s parated from Amphibia and Fishes. . Thus the classes of the Vertebrata are capable of bmng grouped into three provinces: (I.) tho Ichthyoids ( comprisi~g Fishes and Amphibia), d fined by the pr son co of b~·anch1m at some period of existence, the absence of an am.nwn, the absence, or rudimentary development, of the allantois, nucleated blood-corpuscles, and, as will be seen by and by, a parasphenoid in the skull ; (II.) the Sauroids, defined by the absence of branchim at all periods of existence, tho presence of a welldeveloped amnion and allantois, a single occipital condyle, a complex mandibular ramu articulated to the skull by a quadrate bone, nucleated blood-corpuscles and no para phenoid, comprising Reptiles and Bird ; and (III.) the l\Tammals, devoid of branchim and with an amnion and an allantois, but with two occipital condyles and a well-developed basi-occipital and no parasphenoid; a simple mandibular ramus articulated with the squamosal and not with the quadratu1n, with 1nammary glands and with red non-nucleated blood-corpu cles.* These five classes, whether divided into two or three provinces aaain present so many charact rs, ah·eady enumerated, ' b ' • by which they resemble one another, and differ from all other animals, that, by universal consent, they arc a<.hnitted to form the group of VERTEBRATA, which takes its place as one of the primary divisions or ''sub-kingdoms " of tho Animalllingdom. The next four classes-Insecta, Myriapocla, Arachnida, Crustacea- without Joubt also pre ent so many characters in con1mon as to form a very natural assemblage. All are provided with articulated limbs attached to a segment d body-skeleton-the latter, like the skeleton of tho limbs, being an "exoskeleton," or a hardening of that lay r whi h correspoDd with tho outer part of the epidermis of Vertebrates. In all, at any rate in the embryonic condition, the nervous system is con1posed of a double * 'l'o those may be added tlw absence of tho co·rpus caUosum iu tho l.Jrain of Rauroids uud its presence in Munnnals. S o note p. 89. rrHE ARTHROPODA. 75 chajn of ganglia, united by longitudinal commissur .s, and the gullet passes between two of these commis ur s. No one of th members of these four classes is known to possess vibratile cilia. The great majority of these animals havo a distinct heart, provided with valvular apertures, which are in communication with a perivisceral cavity containing corpusculated blood. But tho Cirripedia and the Ostracoda among Crustaceans, and many of the lVIitos among Arachnida, have as yet yielded no trace of distinct circulatory organs, so that tho nature of these organs cannot be taken as a universal character of the larger group 've are seeking ; still less can such a character be found in the . respiratory organs, which vary widely in character, and are often totally absent as distinct structures. Some years ago I endeavoured to show* that a striking uniformity of composition is to be found in the heads of, at any rate, the n1ore highly organized mernbers of these four classes, anrl that, typically, the head of a Crustacean, an Arachnid, a Myria.pod, or an Ins ct is composed of six somites (or segments corresponding with those of the body) and their appendages, the latter being modified so as to serve the purpose of sensory and manducatory organ . I believe this doctrine to be substantially correct; and that, leaving all hypothetical suppositions aside, the head of any animal belonging to these classes may be demonstrated to contain never fewer than four, and never 1nore than six somites with thoir appendages; but, until this view has received confirmation from other workers, I shall not venture to put forward any statement based upon it as part of the definition of the large group or " province " containing the four classes above mentioned, which has received from some naturalists the name of ARTICULATA, from others that of ARTHROPODA the latter being perhaps the more distinctive and better appell~tion. The members of the class Annelida present marked differences from all the Arthropoda, but resemble them in at least one important particular; and that is, the arrangement of the nervous system, whi?h constitutes a ganglionated double chain, traversed at one pmnt by the oosophagus. In almost all other * "On U10 Agamic Roprolluetion and Morplwloo·y of ApMs " T'ransa.ctions 0 .; the L'inn;r.an Society, vol. xxii. b ' 'J |
