62 CONKLIN — EMBRYOLOGY OF A BRACHIOPOD. [April 4, 



4. General Morphology of Larva. — The resemblances between 

 the early embryos of Terebratiilina and those of other meta- 

 zoa are so general in character that they afford little assistance in 

 determining the affinities of the brachiopods. We must therefore 

 rely largely upon the structure of the larva and of the adult for the 

 solution of this problem. 



(a) Comparison with Trochophore. 



Among the chief characteristics of the trochophore larva, as 

 enumerated by Hatschek {^ZZy p. 307), are the following : 



I. (i) Bilateral symmetry, (2) mouth on ventral side, (3) 



anus near posterior end, (4) shape ovoid. 



II. (5) Apical tuft of cilia, (6) preoral ciliated band 



(Trochus), (7) postoral ciliated band (Cingulum), 

 (8) adoral ciliated zone, (9) ventral ciliated furrow be- 

 tween mouth and anus, (10) small cilia over general 

 surface of larva. 



III. (11) Epithelial nervous system, (12) apical plate {Schei- 

 telplatte), ganglion and sense organs, (13) oesophageal 

 nerves and buccal (ventral) ganglion, (14) ventral (and 

 sometimes dorsal) longitudinal nerves. 



IV. (15) Alimentary canal (oesophagus, stomach and intes- 

 tine) horseshoe-shaped and ciliated throughout, (16) 

 stomach retort-shaped, (17) intestine reaches to poste- 

 rior end of body. 



V. (18) Mesoderm partly mesenchymatous, partly epithel- 



ial, (19) mesenchyme gives rise to branched connective 

 tissue cells and thread-like or branched muscle cells, 

 (20) ventral and dorsal longitudinal muscle pairs, (21) 

 preoral and postoral ring muscles, (22) dilatators and 

 constrictors of oesophagus and intestine, (23) meso- 

 thelium gives rise to the paired protonephridium, which 

 is a longitudinal ciliated tube closed at the anterior end 

 by terminal cells, and opening posteriorly on the ven- 

 tral side in front of the anus, (24) paired coelomic sacs 

 at the posterior end. 

 Of these characteristics, numbers 5, 6, 7, 8, 11, 12, 13, 15, 16, 

 17 and 23 are undoubtedly the most important, and all of these 

 except the last are found also in brachiopod larvae. All inves- 

 tigators of the embryology of brachiopods have described the 

 apical tuft of cilia (5) ; the preoral ciliated band (6) is probably 



