. Text-book of embryology. Embryology. 154 INVEETEBRATA CHAP. As we have already hinted, however, in many of the Sabelliformia 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
. Text-book of embryology. Embryology. 154 INVEETEBRATA CHAP. As we have already hinted, however, in many of the Sabelliformia 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, , anal vesicle ; ap, apical plate ; end, endoderm ; M, mother cell of mesoderm lying in endoderm ; , prototroch ; , protonephridium ; st, stomach ; , 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
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