358 



ZOOLOGY 



drawn out into a slit which eventually becomes closed up behind, 

 the anterior portion alone remaining open to form the mouth. 

 The anus is developed later as an invagination in the position 

 of the posterior part of the former blastopore. The mesoderm 

 arises from cells budded off from the endoderm. The prosocoele 

 and mesocoele arise by the formation of fissures ; ' the metaccele 

 by a process of folding off from the archenteron. A large pre- 

 oral lobe is formed, and the anus becomes surrounded by a circlet 

 of cilia (Fig. 288, A). The part of the body on which the anus 



ef.v 



Fig. 287. — Phoronis, transverse section towards the anterior..end. of. v. afferent blood-vessel ; 

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

 cular layer of muscle ; hi, m. mesenteries ; we. /. funnel-like opening of nephridium ; os. 

 oesophagus ; r. rectum. (After Benham.) 



is situated becomes elevated into a conspicuous process. Behind 

 the mouth there is a circlet of cilia, and from this region grow 

 out a circlet of processes — 'the rudiments of the larval tentacles 

 (B). The larva has now reached the stage to which the term 

 actinotrocha is applied. It has a large hood-like lobe overhang- 

 ing the mouth and a circlet of ciliated larval tentacles ; the 

 anus is situated on a prominent process. 



There is a pair of larval excretory organs corresponding to those 

 of the trochophore larva (p. 322) : these apparently do not 

 become converted into the nephridia of the adult. A thickening 



