350 



ZOOLOGY 



SECT. 



The posterior, and by far the most extensive part of the ccelome, 

 occupies the whole of the length of the body behind the trans- 

 verse partition. It is subdivided into two by a median longi- 

 tudinal mesentery (Fig. 292 ; ra., ra.), which extends from the oral 

 to the anal surface and supports both limbs of the alimentary 

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

 mesentery extending from the body-wall to the oesophagus (ce.} 

 in the one compartment (usually termed the right), and to the 

 rectum (r.) in the other (left). The alimentary canal is bent on 

 itself to form a loop, as in the Polyzoa : it is distinguishable into 



et ~ v 



m 



. 



~** 



m 



FIG. 292. Phoronis, transverse section towards the anterior end. af. v. afferent blood-vessel ; 

 c. TO. circular layer of muscular fibres ; ef. v. efferent blood-vessel ; ep. epidermis ; c. TO. cir- 

 cular layer of muacle ; TO., TO. mesenteries ; ne. f. funnel-like opening of nephridium ; 

 ce. oesophagus ; r. rectum. (After Benham.) 



cesophageal, gastric and intestinal regions. There is a closed system 

 of blood-vessels with contractile walls containing red blood-corpuscles. 

 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 between mouth and anus, and giving oft' 

 nerves to the tentacles. There are no organs of special sense. 

 Phoronis is hermaphrodite. Ova and sperms are developed in 



