GENERAL CONSIDERATIONS ON THE MOLLUSCOIDA. 87 



the foot of the Molhisca. Although, from the position of the organs, 

 such a comparison is admissible, yet there do not appear to be 

 sufficient grounds for this liomology. 



The SipuncuUdae show remarkable structural agreement with the 

 Plioronidae, but this similarity may possibly be explained by a similar 

 manner of life, and does not necessitate the assumption of a nearer 

 relationship between the two groups. The larva of the Sipuncididae 

 is closely allied to the Annelidan larva and shows the true Trocho- 

 phore type, from which the Actinotrocha and the other Molluscoidan 

 larvae are to some extent removed. It ought specially to be pointed 

 out that the Sipuncididae agree with the Mollusca and the Annelida 

 in the presence of two primitive mesoderm -cells which produce 

 paired mesoderm-bands, while, in the Molluscoida, the origin of the 

 mesoderm from the archenteron through folding is common. The 

 rise of the coelomic sacs from the archenteron through folding is 

 most clearly to be recognised in the Brachiopoda, Ijut, in the 

 Phylactolaemata also, the coelom must be understood to develop 

 in this way, and the statements of Caldwbll as to the formation 

 of the mesoderm in Phoroms are favourable to such a view, although 

 the observations of Roule do not appear to support it. Although 

 we, as stated above in describing the development of the Chaetog- 

 natha, are not inclined to lay great weight on these differences in 

 the formation of the mesoderm, the facts must be pointed out. 



The shifting forward of the anus and the consequent shortening 

 of the dorsal area lying between the mouth and the anus is brought 

 about differently in the Sipimculidae (Vol. i., p. 363) and in Phoronis. 

 In the SipuncuUdae, the shifting of the anus is a consequence of 

 very gradual alterations brought about by growth. The invagination 

 which gives rise to the posterior end of the body in Phoronis is 

 here altogether wanting. The principal point on which stress must 

 be laid in comparing Phoronis and the SipuncuUdae is the fact that 

 the circle of tentacles surrounding the mouth has a different mode 

 of origin in the two groups, so that the tentacles cannot be con- 

 sidered as homologous. In Plioronis, this row of tentacles can be 

 traced back to a formation of lobes in the region of the post-oral 

 ciliated ring. In the Sipunculid larva, on the contrary, according 

 to Hatschek (IS^o. 5 ; cf. Vol. i., p. 363), the post-oraL ciliated ring 

 has nothing to do with the origin of the tentacles which surround 

 the mouth. The post-oral ciliated ring of the Sipunculid larva lies 

 still further back, and corresponds to the anterior edge of the 

 sphincter, which, after the proboscis-like anterior part of the head 



