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PHYLUM MOLLUSCOIDA 



357 



an nepkr ne P h 



Fig. 280.— Phoronis australis, free end, 

 magnified, an. anus ; ep. epistome ; mo. mouth ; 

 nephr. nephridial aperture ; neph. nephridium ; 

 (After Benham.) 



canal ; and each of these is further subdivided by a longitudinal 

 mesentery extending from the body-wall to the oesophagus (os) 

 in the one compartment 



(usually termed the right), ,«t> 



and to the rectum (r) in 

 the other (left). The ali- 

 mentary canal is bent on 

 itself to form a loop, as in 

 the Polyzoa: it is distinguish- 

 able into oesophageal, gastric 

 and intestinal regions. There 

 is a closed system of blood- 

 vessels with contractile walls 

 containing red blood-cor- 

 puscles. The nervous system 

 lies immediately below the 

 cells of the epidermis. Nerve- 

 elements are generally distributed over the surface, but are 

 specially concentrated in the form of a ring surrounding the 

 body just behind the mouth, but not enclosing the anus, 



thickened into a ganglion be- 

 tween mouth and anus, and 

 giving off nerves to the ten- 

 tacles. There are no organs of 

 special sense. 



Phoronis is hermaphrodite. 

 Ova and sperms are developed 

 in the coelome towards the pos- 

 terior end 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 tentacles, 

 where the ova are impregnated 

 ( — according to another account, 

 fertilisation takes place in the 

 coelome — ), and they go through 

 the early stages of development 

 fixed to the tentacles. The 

 segmentation is complete and 

 slightly unequal : when four blas- 

 tomeres are formed two larger, 

 darker endoderm and two smaller, 

 clearer ectoderm cells are to be 

 distinguished. A blastula is 

 formed with clearer ectoderm cells on one side ; invagination 

 takes place; and, as the embryo elongates, the blastopore is 



In.. 886.— Phoronis australis, internal 

 organisation. a/. W. afferent blood vessel ; 

 an. anus ; ef.bl. efferent blood vessel ; ep. 

 epistome ; me*, mesentery ; mo. mouth ; 

 iitphr p. uepbridiopore ; nephr. d. duct 

 of nephridium ; nephrost. nepbrostome 

 (internal opening of nephridium ; ce». oeso- 

 phagus ; red. rectum ; red. met. recta.1 

 mesentery ; gjpt. septum ; tent, tentacles 

 (cut short). (After Benham.) 



