ON THE DIPLOOHOKDA. 403 



3. The first pair of post-oral elements (collar-somites) are 

 formed of masses of cells segmented from the distal ends of 

 the hypoblast cells vvhicli lie opposite to slight depressions 

 in the archenteric walls. These cells at first have no cavity. 



4. The second pair of post-oral elements (trunk-somites) 

 arise at the anal extremity in a similar way to the former, but 

 they may consist of invaginations of hypoblast in which there 

 are two walls in close contact. Under any circumstances 

 these mesoblastic elements come to lie between hypoblast and 

 epiblast as paired masses with no cavity. 



As regards 1, there is an identity in type with Balano- 

 glossus. 



2. The development of the pre-oral ccelom in Torn aria 

 does not appear to have been followed except in the case of 

 Goette's larva, in which it is indicated as in course of inva- 

 gination from the gut. Morgan (14a), however, has thrown 

 doubt upon this interpretation. In any case the pre-oral 

 coelom arises much earlier in Tornaria than the rest of the 

 mesoblast, and this agrees exactly with Ph or on is, as has 

 been shown above. 



In the case of the demersal larva of Bateson (la) the data 

 are definite, and the pre-oral ccelom appears to arise in a 

 manner precisely similar to that of Phoronis. In both 

 cases it commences to be separated from the archenteron at 

 the time when the proboscis on the one hand, and the pre- 

 oral hood on the other, are commencing to be differentiated 

 externally. In both cases this coelomic pouch, after separa- 

 tion from the archenteron, grows backwards laterally and 

 dorsally, forming a pair of lateral horns. '^ The pouch of 

 mesoblast grows backwards, surrounding the gut except on 

 the ventral surface, but especially forming the hollow horns 

 lying in a horizontal position, one on each side of the gut" 

 (Bateson, loc. cit., p. 220). 



The only difference appears to lie in the earlier separation 

 of the ccelomic pouch from the archenteron in Phoronis, 

 and in this feature it resembles Tornaria. 



3. The development of the collar-somites (mesocoeles) in 



