CHAPTER XVII. 



BRACHIOPODA. 



OUR knowledge of the ontogeny of the Brachiopoda is still somewhat 

 incomplete, especially with regard to certain important points. The 

 anatomy of the adult forms is better known, but the close crowding 

 of the organs between the shell-valves in the larva renders it difficult 

 to ascertain their relative positions. This very crowding has no 

 doubt been the cause of many of the changes that have taken place 

 in the original type of organisation. All that is known of the 

 ontogeny of the Brachiopoda, however, points to the conclusion that 

 they are closely related to the two groups which have just been 

 considered the Plioronidae and the Ectoproctous Bryozoa. This 

 view is founded upon the presence of a tentacle-bearing lophophore, 

 originally horse-shoe-shaped, and of an integumental fold (epistome) 

 above the mouth, and, further, on the agreement that prevails in the 

 three groups with respect to the body-cavity and the nephridial 

 system. The characteristic pelagic larvae of the Brachiopoda also, 

 can without difficulty be brought into agreement with those of the 

 Ectoprocta. 



In our description of the Brachiopoda we shall treat of the 

 Testicardines and the Ecardines separately, beginning with the first of 

 these groups, the ontogeny of which has been more fully investigated. 



1. Testicardines. 

 A. Embryonic Development. 



The first ontogenetic stages of this group have been investigated 

 by LACAZE-DUTHIERS (No. 10 in Thecidium), MORSE (Nos. 11 and 12 

 in Terebratulina), and especially by KOWALEVSKY (No. 8 in Argiope, 

 Thecidium, Terebratula, and Terelratulina). The more recent 

 researches of SHIPLEY (No. 16) have, in all essential points, confirmed 

 the statements of KOWALEVSKY. 



With respect to the first ontogenetic processes, the Testicardines 



