VIII 



PHYLUM MOLLUSCOIDA 



351 



the ccelome towards the posterior 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 ccelome ), and they, go through the early stages 

 of development fixed to the tentacles. The segmentation is com- 

 plete and slightly unequal : when four blastomeres 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 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 meso- 

 derm arises from cells budded 

 off from the endoderm. The 

 prosocosle and mesocoele arise 

 by the formation of fissures ; 

 the metaccele by a process of 

 folding off from the archen- 

 teron. A large pre-oral lobe is 

 formed, and the anus becomes 

 surrounded by a circlet of cilia 

 (Fig. 293, t. tr.). The part of 

 the body on which the anus 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 (Fig. 294, B). The 

 larva has now reached the stage 

 to which the term actinotrocha is applied. It has a large hood-like lobe 

 overhanging 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 ; these have no ccelomic apertures, but 

 are provided with solenocytes (see Section X, Annulata). They 

 apparently do not become converted into the nephridia of the 

 adult. A thickening of the ectoderm of the pre-oral lobe, sometimes 

 bearing eyespots, appears to represent the apical plate of the 

 trochophore. At the point where the oesophagus opens into it, 

 the gastric region of the alimentary canal gives off forwards in one 



FIG. 293.- Actinotrocha larva of Phoronis, 

 lateral view. ap. apical plate; l.v. blood- 

 vessel ; m. mouth ; m. tr. circlet of cilia and 

 tentacles ; t. tr. circlet of cilia round anus. 

 (From MacBride, after Metschuikoff ) 



