822 A. T. MASTERMAN. 



trocha is more nearly allied to Balanoglossus than to its 

 larval form, Torn aria. Thus a closer agreement would pro- 

 bably be found between the latter and an earlier stage of 

 Actinotrocha than is here described. There are, however, 

 certain general points of similarity which bear out the con- 

 clusions already arrived at in the former comparison of 

 Balanoglossus and Actinotrocha, 



Each has a mouth overhung by a large pre-oral hood, at the 

 posterior mid -dorsal border of which is a thickened nervous 

 area. From this runs a pre-oral ciliated band round the edge 

 of the hood, and a post-oral band behind the mouth. In 

 Tornaria the bands are more convoluted, but the arrangement 

 is well seen in Stage iv, figured by Morgan (17). The terminal 

 anus is surrounded by a powerful perianal band, which has 

 long lash-like cilia, and forms the main locomotive organ of 

 the body (cf. 26). 



The condition of the coelom is similar, though Tornaria is 

 less developed in this respect. It, however, shows an unpaired 

 pre-oral ccelom with a dorsal pore, a pair of collar coeloms, and 

 a pair of trunk cavities. In the course of development the 

 trunk region is in each case greatly elongated. 



(3) (The Comparison of Ph or on is with Balanoglossus 

 must be left to another Paper.^) 



We may now inquire if the acknowledgment of Phoronis 

 as closely connected with the chordate stock sheds any further 

 light upon the origin of the Chord at a, or of organs peculiar 

 to this group. 



As regards the notochord a very primitive condition is met 

 with. In Actinotrocha the notochords are little more 

 than vacuolated areas of the gut wall. Their lumen is in 

 continuity with that of the gut, and the process of vacuolisation 

 is very simple and instructive. Each cell can be observed to 

 be gradually filled with turgid vacuoles, and these are not 



1 We may state that the dififerences in habitat and their resulting modifi- 

 cations obscure most resemblances in protomere and mesomere, but that the 

 arrangement of the parts in the metamere or trunk is closely similar. (See 24.) 



