XII 



PHYLUM MOLLUSCA 



715 



from fourteen to eighty. The mouth and anus are both median, 

 situated at the anterior and posterior extremities respectively. 



Alimentary System. — In the Aplacophora the mouth is usually 

 a longitudinal, rarely (O/uviode riiia) a transverse, slit, situated 

 ventrally near the anterior extremity. There is a buccal cavity, 

 with a radula ^ in some cases (Fig. 602. rad), and in others a single 

 chitinous tooth supporting smaller denticles : sometimes teeth are 

 entirely absent. There are both salivary and buccal glands. 

 Very characteristic of the group as compared with other Molluscs 

 is the presence of a straight intestine devoid of coils, and having 

 connected with it either a single cascum or 

 a double row of c£eca. In the Placophora 

 the buccal cavity always contains a well- 

 developed odontophore and radula. The 

 intestine is elongated and coiled. There 

 are salivary glands and a large paired 

 liver (Fig. (J03, liv.). 



Body-Cavity. — In the Aplacophora the 

 interstices between the organs and the 

 body-wall are filled with a form of con- 

 nective-tissue with muscular fibres ; a 

 vertical diaphragm (Fig. 602, dice.) separates 

 the posterior part of the body, contain- 

 ing the pericardium (peri), from the 

 rest. In the Placophora the ccelome 

 (Fig. 603) is an extensive cavity, lined 

 with a ccelomic epithelium, and divided 

 into three completely separated parts — • 

 the pericardium, the genital cavity, and 

 the general body-cavity. 



Vascular System. — The vascular sys- 

 tem of the Aplacophora is very rudi- 

 mentary. There is a heart enclosed in a 

 pericardium, and composed, when best 

 developed, of an auricle and a ventricle (Figs. 602 and 605, i^cri). 



In Chiton there is a well-developed heart (Fig. 603, hL) consist- 

 ing of a median ventricle and two lateral auricles. The pericardial 

 cavity in which it lies is a space of considerable extent in the 

 posterior region of the body, below the two last valves of the 

 shell. 



The Nervous System consists in the Aplacophora (Fig. 604, 

 A,B,C) of four longitudinal nerve-cords — two pedal and two pa/Ha!. 

 These are connected together by an a3sophageal ring, thickened 

 dorsallj' into a single or double cerebral ganglion ; and in front of 

 this is a second, more slender storaatorjastric nerve-ring with small 



' For a description of the stiiicture of this characteristic organ, sue the 

 account of Triton (p. 72(j). 



z z 2 



Fig. 001.— Chiton, valves of 

 shell. (Fruin the Cambridge 

 Natural Hiiloi-y.) 



