VIII 



PHYLUM MOLLUSCOIDA 



:529 



mo 



tent 



the cavity of the body into three chambers. The alimentary 

 canal (Fig. 263) is divisible into cesophageal, gastric, and intestinal 

 regions There is a closed 

 system of blood-vessels 

 with contractile walls con- 

 taining red blood-corpus- 

 cles. The nervous system 

 is not completely separated 

 from the epidermis. Its 

 central part consists of a 

 horseshoe - shaped post- 

 oral nerve-ring at the 

 base of the tentacles, in 

 front of which lie two cili- 



1 , FIG. 2(52. Phoronis australis, free end, 



ated grooves, apparently magnified. n,>. anus; ^. epistome; >. mouth; 



organs of sense. Collections ^ Sam!) 1 aperture ; "^ nephri< 



of nerve-fibres and gan- 

 glion cells lie in the integument and are connected with 

 nervous processes of the epidermal cells. 



Phoronis is hermaphrodite. Ova and sperms are developed 



in the anterior left chamber of 

 the ccelome from cells on the 

 wall of one of the large blood 

 vessels. When mature these pass 

 out through the nephridia to 

 the spaces enclosed by the ten- 

 tacles, where the ova are im- 

 pregnated (or, according to 

 another account, fertilisation 

 takes place in the ccelome) and 

 go through the early stages of 

 their development fixed to the 

 tentacles. The segmentation is 

 complete and slightly irregular ; 

 when four blastomeres are formed 

 two larger, darker, endoderm, 

 and two smaller, clearer, ecto- 

 derm cells are to be distin- 

 guished. A blastula is formed 

 with clearer ectoderm cells on 

 one side ; invagination takes 

 place; and, as the embryo elon- 

 gates, the blastopore is drawn 

 out into a slit which eventually 

 becomes closed up behind, the 

 anterior portion alone persisting and giving rise to the mouth. 

 The cells bordering upon the posterior part of the blastopore take 



m&s 



FIG. 263. Phoronis australis, internal 

 organisation, af. 1>1. afferent blood vessel ; 

 are. anus ; if. bl. efferent blood vessel; *//. 

 epistome ; nut. mesentery ; nw. mouth ; 

 nephr. p. nephridiopore ; nephr. d. duct 

 of nephridium ; iiephrost. nephrostome (in- 

 ternal opening of nephridium ; ees. oeso- 

 phagus ; rtct. rectum ; rect. tncs. rectal 

 mesentery ; sept, septum ; tent, tentacles 

 (cut short). (After Benham.) 



