XII 



PHYLUM MOLLUSCA 



717 



which unites with its fellow of the opposite side : two small ganglia 

 lie in this visceral commissure near the middle line. The large 

 cords contain nerve-cells throughout their length. 



The conspicuous organs of special sense, present on the head of 

 Gastropods (vide infra), are absent in the Placophora, as in the 



peri 



gon. 



hct 



Fr<;. 603. Diagrammatic longitudinal section of Chiton, specially intended to show the 

 relations of the parts of the cuelome, which, except the genital parts, are bordered with a 

 thick line. an. anus ; ent. enteric cavity ; ft. foot ; gon. gonad ; hd. head-lobes ; lit. heart ; 

 lir. liver ; mo. mouth ; neph. nephridium ; peri, pericardial cavity. (From Simroth, after 

 Haller.) 



Aplacophora. A pair of processes situated in front, at the sides of 

 the mouth, have the character of labial palps. In the buccal 

 cavity there are cup-shaped cjustatory organs supplied with 

 nerves from the cerebral commissures, and in front of the 

 odontophore is a thickening of the epithelium the subradular 

 organ containing nerve-endings. Remarkable sensory organs, 

 the micrcesthetes and the megalcesthetes, lie in the canals already 



I 1 



pc 

 D 



Fio. 604. Nervous system of Amphineura. A, Proneotnen'm : B, Ntomenla : C, Chwtoderiiui ; 

 D, Chiton, c, cerebral ganglia ; I, I, pallial cords ; pc. posterior commissure ; .<;, stomatogastric 

 commissure or ring, with ganglia ; v, c, pedal cords. (From the t'limliridfie Natural History, 

 after Hubrecht.) 



mentioned as occurring in the superficial layer of the shell-valves. 

 The megalsesthetes may take the form of eyes, with cornea, 

 lens, pigment-layer, iris, and retina ; in some cases the lens is 

 absent. 



