84 COMPARATIVE ANATOMY 



the lower hemisphere round to one side, the ventral side of 

 the future animal. In some cases — e.g. the limpet — the elon- 

 gated blastopore closes up from behind forwards, only the 

 anterior portion persisting as the mouth, which is then situated 

 close under the ciHated ring on the ventral surface. In other 

 cases the anterior part of the blastopore closes up, and the 

 posterior end persists as the anus (Acmssa). It seems prob- 

 able that in the most primitive condition the blastopore closed 

 up in the middle, its anterior and posterior ends persisting as 

 the mouth and anus respectively, and for the convenience of 

 description we will assume that this is the case. The embryo 

 now escapes from the egg-membranes and swims freely in the 

 water by means of its cihated band. It is obviously a trocho- 

 sphere larva, with a ciliated girdle lying in front of the mouth 

 and dividing the body into an upper or pre-oral and a lower 

 or post-oral hemisphere. The mouth opens just below the 

 ciliated band and leads into a gut which in actual cases 

 generally ends blindly, but eventually establishes another con- 

 nection with the exterior by an outgrowth which meets the 

 epiblast at the posterior pole, fuses with it, and gives rise to 

 the anus. The larva further resembles the trochosphere in 

 possessing a pair of mesoblast bands, and in some forms the 

 resemblance is strengthened by the appearance of a pair of 

 provisional excretory tubules, in the form of a row of perforated 

 cells, ciliated internally, and opening to the exterior by excre- 

 tory pores situated on the ventral sides of the body just behind 

 the velum. The position of these provisional excretory tubes, 

 however, is different from that of the pronephridia of the 

 Annelid trochosphere, and most authors describe them as 

 arising from the mesoblast, whereas in Annehds they arise 

 from ectoderm cells. 



In MoUuscan terminology, the upper hemisphere of the 

 larva is called the velum ; the pre-oral ciliated ring is the velar 

 ring, and the whole larva is more usually called a veliger than 

 a trochosphere. It is only in a few cases, such as the limpet, 

 that a really Annelid-like trochosphere is developed. In most 

 instances the similarity is to some extent masked by the pre- 

 cocious appearance of two very characteristic Molluscan organs, 

 the foot and the shell-gland. The former is indicated by a 

 projection in the mid-ventral line between the mouth and the 

 anus. It therefore occupies the place of the central part of 



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