XII 



PHYLUM MOLLUSCA 



715 



a 



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 (Gkcetoderma) a transverse, slit, situated 

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

 with a radula l 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 caecum or 

 a double row of caeca. 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. (303, 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, diet.) separates 

 the posterior part of the body, contain- 

 ing the pericardium (peri), from the 

 rest. In the Placophora the co3lome 

 (Fig. 603) is an extensive cavity, lined 

 with a coelomic 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, peri). 



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

 ing of a median ventricle and two lateral auricles. The pericardia! 

 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 f ) of four longitudinal nerve-cords two pedal and two partial. 

 These are connected together by an cesophageal ring, thickened 

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

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



1 For a description of the structure of this characteristic organ, see the 

 account of Triton (p. 7*26). 



z z 2 



t 



cl 



FIG. 601. Chiton, valves of 

 shell. (From the Camfrridyv 

 Natural history.) 



