350 



ZOOLOGY 



SECT. 



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

 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, m. 5 m.), 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 



ef.u 



m 



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

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

 cular layer of muscle ; m., m. mesenteries ; ne. f. funnel-like opening of nephridium ; 

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



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

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

 The nervous system lies immediately below the cells of the epidermis. 

 Nerve-elements are generally cfotributed 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 off 

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

 Phoronis is hermaphrodite. Ova and sperms are developed in 



