668 



ZOOLOGY 



SECT. 



-peri- 



to the mantle and the ctenidia, are united together by a supra- 

 rectal commissure at the posterior end of the body. Just behind 

 its origin each pallial cord gives off a slender visceral commissure, 

 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 

 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 com- 

 <yon> missures, and in front of the 



odontophore is a thickening 

 of the epithelium the sul- 

 radular organ - - containing 

 nerve - endings. Remarkable 

 sensory organs, the micra'stlictcs 

 and the megalcestJietes, lie in 

 the canals already mentioned 

 as occurring in the superficial 

 layer of the shell valves. The 

 megalffisthetes may take the 

 form of eyes, with cornea, lens, 

 pigment layer with iris, and 

 retina ; in some cases the lens 

 in absent. 



Reproductive and Renal 

 Organs. In the Placophora 

 the sexes are distinct : in the 

 Aplacophora, with the excep- 

 tion of Chretoderma, they are 

 united. In the Aplacophora 

 (Fig. 568), with the exception 

 of Chaetoderma, the gonads 

 are paired. The sexual products pass into the pericardial cavity 

 and thence are carried to the exterior by a pair of ducts (which 

 may be of the nature of nephridia) opening into the cloaca. 



In the Placophora (Fig. 560) there are two symmetrical 

 nephridia opening internally into the pericardium by a ciliated 

 funnel-like opening (n. peri. ji). and opening on the exterior 

 (neph. ap) between two of the posterior ctenidia. Each consists 

 of a looped main tube, into which open numerous minute 

 tubules which ramify among the viscera. The testis and ovary 

 (gori) are similar in appearance, differing only in colour when the 



Fin. 508. Neomenia carinata, reproduc- 

 tive organs; cop. copulatory organs; <//(. 

 gonads enclosed in extensions of the peri- 

 cardial cavity; (tonoil. gonoducts ; peri, peri- 

 cardium ; rect. receptaculum seminis. (From 

 fcsiniroth, after Wiren.) 



