400 ARTHUR T. MASTERMAN. 



The rest of the coelom he derived mainly from a pair of 

 posterior pouches which arose in connection with the 

 "posterior pit" — a remnant of the posterior part of the 

 blastopore. Schultze emphatically denies the existence of 

 these posterior pouches, and in this, as far as P. Buskii 

 goes, I can corroborate him. Many weeks' search amongst 

 hundreds of embryos whole and in section has failed to bring 

 to light a single structure bearing the slightest i-esemblance 

 to either the posterior pit or the posterior coelomic pouches. 

 Schultze (16) suggests that Caldwell was misled by the 

 ventral invagination which gives rise to the adult trunk, but 

 this cannot be the case, as this organ does not appear till 

 long after the larvae have left the parent. 



Caldwell (3) figures these appearances not only in P. 

 Kowalevskii but in P. australis as well, and although I 

 have failed, to find them in the latter I merely state the fact, 

 and defer further consideration till I am able to examine the 

 other species with fresh material. 



Comparison with Development of Balanoglossus. 



In the first of this series of papers on the Diplochorda 

 the structure of the late stage of Actinotrocha was com- 

 pared in some detail with that of Balanoglossus. The 

 facts above recorded of the early development serve to 

 emphasise this comparison. There are at present known 

 at least two types of development in the Enteropneusta, the 

 direct development of Balanoglossus Kowalevskii (?) 

 known to us through the work of Bateson (la), and the so- 

 called indirect development with Torn aria larva. The 

 early stages of this indirect development do not appear to 

 be known, though one would expect to find the development 

 of Phoronis with its free-swimming larva Actinotrocha 

 conform more closely to this type. However, taking Bate- 

 son's type for comparison we find that the stages of seg- 

 mentation are identical. In both cases there is total seg- 

 mentation with very slightly pronounced inequality in the 



