VIII 



PHYLUM MOLLUSCOIDA 



357 



an 



FIG. 285. Fhoronis aus trails, free end, 

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

 nephr. nephi-idial aperture ; neph. nephridium ; 

 (After Benham.) 



ten 



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

 mesentery extending from the body-wall to the oesophagus (<z) 

 in the one compartment 

 (usually termed the right), 

 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 cesophageal, 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 ccelome 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 



Fi< . 2>". Phoronis aus trails, internal 

 organ isation. /. W. afferent blood vessel ; 

 an. anus ; ef. l>l. efferent blood vessel ; ep. 

 epistome ; meg. mesentery ; mo. mouth ; 

 ntl>lt,- p. nephridiopore ; nephr. d. duct 

 of nephridium ; nephrott. nephrostome 

 (internal opening of nephridium ; as. o?so- 

 phagus ; rect. rectum ; rect. mes. rectal 

 mesentery ; sept, septum ; tent, tentacles 

 r Ben 



(cut short). (Afte 



ham.) 



