402 ARTHUR T. MASTERMAN. 



is present in each case a ring of cilia. Such a migration 

 would be very hard to follow, except by direct observation 

 of a developing embryo, in such a type as Phoronis. The 

 larva of Balanoglossus Kowalevskii differs from Tor- 

 naria in developing the perianal band very early alone, 

 whereas the latter develops this band after the post-oral 

 band. In this respect Actinotrocha resembles the free- 

 swimming Tornaria, as the perianal band has not ap- 

 peared in P. Buskii even in the larva with three pairs of 

 tentacles. 



I hope to show later elsewhere that in Tornaria and 

 other larvae, such as those of Echinoderms, the ciliated bands 

 can be classified by their primary functions into trophic and 

 motor. Of these the post-oral band of the Echinoderms, 

 Tornaria, and Actinotrocha is trophic, whereas the peri- 

 anal band of Tornaria, Actinotrocha, and the circular 

 bands of Auricularia and Antedon are motor. Thus in 

 Tornaria and Actinotrocha, with free larval life at a 

 very early stage, the trophic bands are essential, whilst the 

 perianal motor band is added in due course. In the de- 

 mersal type of Balanoglossus Kowalevskii the mouth 

 and anus are closed till late ; no pelagic food is required, and 

 the post-oral band does not appear, whereas the perianal band 

 is developed early for the locomotion of the larva. The same 

 considerations apply to Antedon and pupal Auricularia. 



After closure of the blastopore the next essential com- 

 parison is in the development of the mesoblast. In Pho- 

 ronis, as we have already seen, there has been a great 

 difference of opinion upon this point. The following facts 

 appear to be indicated. 



1. The mesoblast arises from five separate parts of the 

 archenteric hypoblast, of which one is pre-oral and unpaired, 

 and the other four are paired and post-oral. 



2. The pre-oral mesoblast is in the form of a portion of 

 the archenteron which is pinched off from it, the cavity 

 being part of the archenteric cavity shut off in course of 

 development. 



