

ARTICULATA. 



245 



vitality corresponds to the distribution of the 

 nervous system into as many centres as there 

 are corporeal segments."* With respect to the 

 agreement between the number of segments of 

 the body and the ganglions of the nervous sys- 

 tem, it must be observed that in the higher 

 crustaceans, arachnidans, and insects, the gan- 

 glions, though originally as numerous as the 

 segments, subsequently become concentrated 

 by progressive development into masses which 

 are fewer in number, and that also in some of the 

 lowest annelidans, as the leech-tribe, the ex- 

 ternal segments are more numerous than the 

 internal ganglions. 



In many of the molluscous class two nervous 

 cords proceed backwards from the supraoaso- 

 phageal ganglion or brain, and are afterwards 

 brought into communication by ganglionic 

 masses on the ventral aspect of the body ; but 

 in the Articulata the uniting ganglions are 

 always confined to the mesial line of the body, 

 are perfectly symmetrical in their arrangement, 

 and are accompanied by a symmetrical or bila- 

 teral form of the whole body. It is this homo- 

 gangliate disposition of the nervous system 

 which essentially distinguishes the Articulate 

 from the Molluscous and other divisions of the 

 Animal Kingdom, and it is an infallible guide 

 to the true affinities of the classes possessing 

 it. The Cirripeda present a striking example 

 of this fact : these animals, on account of their 

 inarticulate body enveloped in a fleshy mantle 

 and protected by a multivalve shell, were for 

 a long time classed with the mollusca : but 

 the views of those naturalists who considered 

 that they had closer relations to the Arti- 

 culata, although that opinion was founded on 

 a knowledge of their nervous system only, has 

 since been corroborated by every additional 

 fact which has been discovered respecting 

 them. Latreille, in his " Families Naturelles 

 du llegne Animal," first placed the cirripeds in 

 the Articulate series, but being guided by their 

 adult organization, and supposing that they 

 were deficient in visual organs, and underwent 

 no metamorphosis, he joined them with the an- 

 nelidans, to form a division of Articulate ani- 

 mals, " Elminthoida," distinct from the " Con- 

 dylopeda" which include the insects, arach- 

 nidans, and crustaceans, or the Articulata with 

 jointed feet. The later researches of Mr. I. V. 

 Thompson and Dr. Burmeisterf have proved 

 that in the immature state the Cirripeds un- 



members, and as their nervous cords are sim- 

 ple and not brought into communication by a 

 regular series of ganglions, we prefer to leave 

 the Rotifera and Ccelelmintha with the Entozoa 

 and Echinodermata, as a separate and higher 

 subdivision of Cuvier's Radiata, and thus pre- 

 serve the Articulata as a distinct and well de- 

 fined subkingdom, characterized by a dispersion 

 of the nervous system in a series of ganglions, 

 symmetrically arranged and brought into com- 

 munication by a double nervous cord ; by an 

 articulate or jointed structure of the body or its 

 appendages, by the lateral position and hori- 

 zontal movements of the jaws, when these are 

 present, and by the presence of distinct respi- 

 ratory organs. The subdivisions of this sub- 

 kingdom are not founded on the modifications 

 of any single system, but principally rest on 

 the conditions of the sanguiferous and respira- 

 tory organs, in connexion with exterior form, 

 modes of locomotion and generation. 



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 branchiae 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 



mat m me immature state the Cirripeds un- segments, of which the first, called the head, 

 dergo repeated metamorphoses or moults ; that scarcely differs from the others, except by 



the presence of the mouth and of the principal 

 organs of the senses. Many possess branchije, 

 arranged the whole length of the body, or situ- 

 ated at the middle ; others, which for the most 

 part inhabit tubes, have the branchise 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 



they move freely in the water by means of 

 setiferous articulated members, and during this 

 period guide their wanderings by the aid of 

 distinctly developed, though simple eyes. 



Besides the cirripeds the higher organized 

 infusoria and intestinal worms have been 

 proposed by some naturalists to be added to 

 the articulate division of Animals : but as they 

 are neither articulated nor possess articulate 



* See his celebrated memoir, " Sur un nouveau 

 rapprochement a etablir entre les classes qui com- 

 posent le regne animal." " Anwiles du Museum 

 d'Histoire Naturelle, 4to, torn. xix. p. 73. 



t Beitnige zur Naturgeschichte tier Kaukeufuesser, 

 4to. Berlin, 1834. 



