PHYLUM MOLLUSCA 



717 



w 



hich 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 



pen 



hd. 



PIG. 603. Diagrammatic longitudinal section of Chiton, specially intended to show the 

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

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

 /<>. liver; mo. mouth; neph. nephridium ; peri, pericardial cavity. (From Simroth, after 

 Haller.) 



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

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

 jcavity there are cup-shaped gustatory organs supplied with 

 nerves from the cerebral commissures, and in front of the 

 pdontophore 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 



pc 

 D 



FK;. 004. Nervous system of Amphineura. A, Proneomenia : B, Ntomenia ; C, Chatoderma ; 

 1), Chiton, c, cerebral ganglia ; I, I, pallial cords ; pc. posterior commissure ; s, stomatogastric 

 commissure or ring, with ganglia ; v, v, pedal cords. (From the Cambridge 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. 



