160 ZOOLOGY. 



DIVISION OF THE MOLLUSCA, OR MALAC( >/< >AIMA. 



592. The division of the mollusca is composed, as we 

 have already said, of a considerable number of animals which 

 have neither a cerebro-spinal system nor an internal skeleton, 

 and which resemble in these respects the articulated animals, 

 but not having, as these have, the body divided into rings, 

 nor the ganglions (nervous) reunited into a long median chain 

 placed on the ventral aspect of the body. They are distin- 

 guished also from zoophytes by the disposition of the organs 

 of relation being arranged in pairs, and in general they have 

 the mouth and anus more or less close to each other. More- 

 over, they differ much from each other, and are divided into 

 two principal series, namely, the mollusca properly so called, 

 and the molluscoides or tunicata. 



SUB-DlVISlON OF THE MOLLUSCA, 

 PROPEELY SO CALLED. 



I 



593. In this group the nervous system is always com- 

 posed of several ganglions, reunited by medullary cord- 

 to form a sort of double collar, more or less closely around 

 the gullet, but not prolonged posteriorly like a sub-intestinal 

 chain, as in the annulated animals. 



The general form of these mollusca is extremely variable. 

 Their body is always soft, and in a small number only (the 

 sepia, for example) there exists in the interior some solid 

 non-articulated pieces, serving rather to protect the viscera 

 than to furnish to the locomotory apparatus levers and points 

 of support. The muscles are fixed directly into the integu- 

 ments, and seem to act only on the point into which they are 

 inserted ; and thus the movements of the animal are slow 

 and ill directed. In a small number of these beings (the 

 sepia, for example) there are flexible and elongated appendages 

 (Fig. 162), intended for locomotion, but in most instances the 

 animal cannot displace itself but by successive contractions of 

 various points of the lower surface of its body; and even 

 when there exist limbs, these organs are reunited in a group 

 at one extremity of the body, and never disj.dvrd in a -\ in- 

 metrical series, as in the vertebrate and articulate animals. 



