xn 



PHYLUM MOLLUSCA 



(599 



on/ 



pert 



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

 rectal commissure at the posterior end of the body. Near its 

 origin each pleural 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- 

 missure, and in front of the 

 odontophore is a thickening of 

 the epithelium the subradular 

 organ containing nerve-end- 

 ings. Remarkable sensory 

 organs, the micrcesthetes and the 

 megalcesthetes, lie in the canals 

 already 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. 



Reproductive and Renal 

 Organs. In the Placophora 

 the sexes are distinct : in the 

 Aplacophora, with the exception 

 of Chsetoderma, they are united. 

 In the Aplacophora (Fig. 615), 



f rn . 



Wltn the exception OI UiiaetO- 



derma, the gonads are paired. 



The sexual products pass into 



the pericardial cavity and thence 



are carried to the exterior by a pair of ducts (coslomoducts) 



opening into the cloaca. Renal organs are unknown in the 



Aplacophora. 



In the Placophora (Fig. 616) there are two symmetrical kidneys, 

 each opening internally into the pericardium by*"a ciliated funnel- 

 like opening (n. peri, ap), and externally (neph. ap) between two 

 of the posterior ctenidia some little distance in front of the anus. 

 Each consists of a looped main tube, into which open numerous 

 minute tubules which ramify among the viscera. The testis and 

 ovary (gon) are similar in appearance, differing only in colour 



Y Y 2 



FIG. 615. Neomenia carinata, reproduc- 

 tive organs, cop. copulatory organs ; gon. 

 gonads enclosed in extensions of the peri- 

 cardial cavity ; gonod. gonoducts ; peri, peri- 

 cardium ; reel, receptaculum seminis. (From 

 Simroth, after Wiren.) 



