Ml 



thvum molu'sca 



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 



hd. 



Fio. 60S.— Diagrammatic longitudinal section of Chiton, specially intended to show the 

 relations of the parts of the crelome, which, except the genital |>arts, arc bordered with a 

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

 lit: liver; ,no. mouth; ntpk. nophridium ; peri, pericardial cavity. (From Siniroth, 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 gustatory _ organs supplied with 

 nerves from the cerebral commissures, and in front of the 

 odontophore is a thickening of the epithelium — the suhradular 

 organ — containing nerve-endings. Remarkable sensory organs, 

 the micrmsthetes and the megalwsthetes, lie in the canals already 



"~* — v^- 



I T 



5 



Fi(i. UOi.— Nervous system of Amphineura. A, Pnmmmenia: B, Ntomenia: C, Chatoderma : 

 D, Chiton, r, cerebral ganglia ; I, I, pallial cords ; pc. posterior commissure ; s, stomatogastric 

 commissure or ring, with ganglia ; v, v, pedal cords. (From the Ctnabridg* Natural History, 

 after Hubrecht.) 



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

 The megala>sthetes may take the form of eyes, with cornea, 

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

 absent. 



