XII PHYLUM MOLTATSCA 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 



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n,eph 



,rt 



ent '''■^ 



Fir. 608.— 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 ; mt. enteric cavity ; ft. foot ; (jon. gonad ; hd. head-lobes ; ht. heart ; 

 IU\ liver; ino. mouth; iiejj/i. nephridiuxn ; yej'i. 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 ai-e cup-shaped (justaiory (irgana supplied with 

 nerves from the cerebral commissures, and in front of the 

 odontophore is a thickening of the epithelium — the snhradulur 

 organ — containing nerve-endings. Remarkable sensory organs, 

 the micrcBSthetes and the megalcesthetes, lie in the canals already 



\\S/ J 1 



Fig. 604. — Nervous system of Amphineura. A, Proneomenia ; B, Ntomcaia : C, Chcetoderma ; 

 J), Chiton, c, cerebral ganglia ; t, I, pallial cords ; pc. posterior commissure ; s, stomatugastric 

 commissure or ring, with ganglia ; v, v, pedal cords. (From the Cambridge Natural H'atory, 

 after Hubrccht.) 



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. 



