1902.J CONKLIN — EMBRYOLOGY OF A BRACHIOPOD. 61 



Very little is certainly known of the formation of the mesoderm 

 and coelom among the Bryozoa. In the Ectoprocta the larval form 

 is usually solid, the coelom and enteron having undergone extensive 

 if not complete degeneration, while in those forms in which the 

 coelom is still preserved its method of origin is highly peculiar. 

 For example, in the Phylactolaemata the central cavity of the em- 

 bryo is generally considered to be the ccelom, although at the stage 

 at which it appears there is neither endoderm nor enteric cavity in 

 the embryo (see Korschelt and Heider's Text-Book). 



Among the Endoprocta the mesoderm arises in Pedicellina, 

 according to Hatschek, from two pole cells which appear at the 

 posterior edge of the blastopore ; these cells by repeated divisions 

 give rise to two short mesoderm bands, and from these bands meso- 

 derm cells arise which fill the space between the ectoderm and the 

 endoderm. It appears therefore that no direct comparison can be 

 made between Terebratidina and the Bryozoa in the matter of the 

 formation of mesoderm and coelom, 



3. Orientation of Embryo and Larva. — As has been pointed out 

 already the relation of the chief axis of the gastrula to the chief 

 axis of the larva is the same as is found in all Heteraxonia (Hat- 

 schek) or Hypogastric forms (Goette). The animal pole of the 

 tgg and the apical pole of the gastrula become the cephalic pole 

 {Hirnfeld) of the larva, while the blastopore comes to lie on the 

 ventral side. Such a relation of the embryonic and larval or adult 

 axes is of very general occurrence, being found at least in all Tro- 

 chozoa (Hatschek). Moreover in having a blastopore which 

 becomes narrow from side to side and then closes from behind for- 

 ward, and also in the formation of the stomodaeum at the place 

 where the blastopore closed, Terebratidina agrees with a large 

 number of bilateral animals belonging to widely different phyla. 

 These characters are so general, therefore, as to be of little value in 

 determining the affinities of the brachiopods. Regarding the 

 apical sense plate as anterior and the suboesophageal sense plate as 

 ventral in position, it follows that the peduncle is posterior and the 

 mantle folds dorsal and ventral ; consequently even after the meta- 

 morphosis the peduncle is posterior, and the valves which are 

 formed by the mantle lolds are dorsal and ventral, while the opening 

 of the valves is anterior. This orientation is the commonly 

 accepted one and is directly opposed to Caldwell's ('82) remark- 

 able views, according to which both valves and the peduncle are 

 ventral in position. 



