ON THE DlPLOOHOJiDA. 321 



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traced from the lateral walls iu a converging manner to the 

 posterior angles, M'herc iu Actinotrocha they are inserted 

 in the '' pre-oral collar" mesentery, and in Balanoglossus in 

 the sub-notochordal skeletal rod. The arrangement of these 

 muscles will be shown to be in Cephalodiscus identical 

 with that of Actinotrocha. In Actinotrocha there 

 are two proboscis pores opening on either side of the sub- 

 neural sinus (fig. 51) in identically the same manner as in 

 Cephalodiscus. In Balanoglossus the pores are some- 

 what further away from the middle line, but in an homologous 

 position. The collar-cavities are closely similar, with a ten- 

 dency in each case to a production of the dorsal part forwards 

 into the neck. The collar pores have already been compared 

 to the collar nephridia of Actinot rocha. The relationships 

 of the collar coelomic walls to the vascular system and the gut 

 are practically identical. 



The trunk cavities are continuous dorsally, aud ventrally 

 they form a mesentery. From their walls are derived in each 

 case the gonads (Phoronis). The front dorsal part of the 

 trunk coelom is produced into a pair of perihsemal spaces, 

 embracing the dorsal blood-vessel. 



We thus see that not only is there in both types a segmen- 

 tation of the coelom into five pouches, one pre-oral and 

 unpaired (leaving out of consideration for the present the 

 possible coelomic value of the proboscis vesicle), and two pairs 

 of post-oral pouches, but these pouches are similar, to a large 

 extent, in their anatomical relationships, though perhaps the 

 presence of tentacles in Actinotrocha tends to make the 

 resemblance to Cephalodiscus even closer. 



Thus a very general and brief comparison of the nervous, 

 alimentary, vascular, skeletal, aud coelomic systems brings out 

 a close agreement, the genetic origin and value of which can 

 hardly be denied. 



(2) Actinotrocha and Tornaria. 



The comparison of these two larvae is interesting in that it 

 leads one to the conclusion that the fully developed Ac tin o- 



