CLASSIFICATION OF ANIMALS. 



13 



gaiigliata, the principal centre of the nervous system bears the form 

 of a ring, surrounding the gullet, from which the nerves 

 radiate, often unsymmetrically, to different parts of the 

 body {fig. 2.) : the brain is represented by ganglions 

 above (a) or at the side {b) and below the gullet ; other 

 ganglions (c d) are developed in other parts of the body. 

 The form of the body corresponds with the disposition of 

 the nervous system, and is commonly unsymmetrical. In 

 a single order {Cephalopods) the muscles originate from 

 an internal rudimental cartilaginous skeleton : in the rest 

 they are attached only to the skin, which forms a soft 

 envelope in which there are developed in many species 

 one or two, rarely more, calcareous plates, called shells. 



The blood is colourless, or not red ; the heart compact, 

 muscular, and propelling the blood through a closed sys- 

 tem of arteries and veins, or venous sinuses. * 



The respiratory organ is never absent ; and, with the 

 exception of one family {Ascidkms), the cavity containing 

 it receives, or opens near, the anus. 



The Mollusca are dioecious or hermaphrodite : some of 

 the lowest organised species are parthenogenetic. 



The third primary division of the animal kingdom, viz. 

 the Articulata, has the brain in the form of a ring, em- 

 bracing the gullet : a double ganglion above the tube 

 supplies the chief organs of sense : from the sub-oesopha- 

 geal ganglions two chords are extended along the ventral 

 surface of the abdomen, and are, in most species, united 

 at certain distances by double ganglions, which give origin 

 to the nerves of the body-segments and their appendages 

 {fig. 3.). From the symmetrical disposition of the nervous 

 centres, I have called this sub-kingdom Homogangliata. The body 

 presents a corresponding symmetrical form. The skeleton is external, 

 and consists of articulated segments, of frequently an annular form : 

 the articulated limbs in those species which possess them have a similar 

 condition of the hard parts, in the form of a sheath, which encloses the 

 muscles. 



The respiratory organs commonly open upon the sides of the body ; 

 rarely near the anus, and never communicate with the mouth. 



* Tlie latter phrase is added, in the p esent edition, because the term " closed 

 system " has been supposed to imply a particular form of the veins, viz. the tubular : 

 and has been objected to by the supporters of the hypothesis that the blood in many 

 Mollusks is diffused in interspaces or lacunas of the viscera (XVI. p. 292.). I have 

 shown, however, that these supposed lacunae are sinuses closed or formed by the 

 proper tunic of the veins. Sec XVI. p. 315. pi. 4., and pi. 1. of the present volume. 



