718 



ZOOLOGY 



Reproductive and Renal Organs. — In the Placophora 

 the sexes are distinct : in the Aplacophora, with the excejjtion 

 of Chajtodenna, they are united. In the Aplacophora (Fig. 60.5), 

 with the exception of Chastoderma, the gonads are paired. 

 The sexual products pass into the pericardial cavity and thence 

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

 opening into the cloaca. Nephridia are unknown in the 

 Aplacophora. 



In the Placophora (Fig. 606) there are two symmetrical 

 nephridia, 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 vis- 

 cera. The testis and ovary {gon) 

 are similar in appearance, dif- 

 fering only in colour when the 

 products ai'e mature. Each is 

 an unpaired sac marked by a 

 series of slight lateral constric- 

 tions. There are two gono- 

 ducts, each opening immedi- 

 ately in front of the corre- 

 sponding nephridial duct. 



Little is known of the 

 development of the Aplaco- 

 phora. The eggs undergo 

 complete segmentation, and 

 give rise to a gastrula by in- 

 vagination. This develops into 

 a form of trochophore with a 

 ciliated ring, the prolotroch.. 

 The larva is provided for 

 of seven calcareous plates on the dorsal 



rec-t 



aonod 



■perv 



in. fiO'..— Neomenia carinata, reproduc- 

 tive organs, cop. copulatory organs ; 17071. 

 gonads enclosed in extensions of the peri- 

 cardial cavity ; gonad, gonoducts ; peri, peri- 

 cardium ; 7v. /. receptaculum seminis. (From 

 Simrotlij after Wiren.) 



a time with a row 

 surface. 



The eggs of Chiton are fertilised in the mantle-cavity, where in 

 one species thej'are retained until the embryos are fully developed. 

 At first the segmentation is tolerably equal — the ovum becoming 

 divided into four approximately equal blastomeres : but at the 

 stage of eight cells, four on one side are to be distinguished 

 as larger than the other four. These two sets undergo further 

 divisions and arrange themselves in such a way as to form 



