154 



INVEETEBRATA 



CHAP. 



As we have already hinted, however, in many of the Sabellif ormia 

 there is a development closely resembling that of Polygordius, and 

 the animal begins its larval life as a true Trochophore. In the genus 

 Eupomatus, according to Shearer (1911), the blastopore divides into 

 primitive mouth and primitive anus — as in Polygordius the mouth 

 opening persists but the primitive anus is closed ; later the permanent 

 anus is formed just where the primitive anus disappeared. In 

 this form, when the Trochophore has begun to swim about, there is 

 as yet no trace of adult mesoderm ; the mother cells of this layer 

 4d'' and 4d' are still situated in the wall of the intestine, and only 

 when the larva has been leading a free-swimming life for a day or 



anv 



Fig. 113. — Figure illustrating the origin of the mother cells of the adult mesoderm in 



Eupomatus. (After Shearer.) 



A, section of young gastrula showing the inward migration of endoderm cells and the formation of 



, the mother cell of the mesoderm hy the division of one of them. B, sagittal section of Trochophore 



larva three days old. 0. sagittal section of hinder end of Trochophore larva, nmch enlarged, an.v, 



anal vesicle ; ap, apical plate ; end, endoderm ; M, mother cell of mesoderm lying in endoderm ; p.tr, 



prototroch ; P.N, protonephridium ; st, stomach ; t.tr, telotroch. 



two do these cells emerge from the gut wall and begin to found the 

 mesoderm bands. Further, in Eupomatus, the two archinephridia, 

 which are formed exactly as in Polygordius by the migration inwards 

 of certain cells of the third quartette and which develop so as to 

 form the only pair of protonephridia present in this larva, persist in 

 the adult, and here constitute the two large effective nephridia 

 which serve as principal excretory organs ; they are situated in the 

 prostomium and open by a dorsal pore. According to Shearer they 

 open near the anus in the larva, but their opening becomes shifted by 

 the growth of the " worm-body." The Trochophore of Eupomatus is 

 further remarkable for developing a large clear vacuole, the so-called 

 anal vesicle (an.v, Figs. 113 and 114), in the cells which support 

 the telotroch. 



