TROCHOPHORE OF HYDROIDES T'XCINATUS (eUTOMATUS). 545 



In the following" account I liave incorporated some drawings 

 and notes of Hydroides pec tin a ta, kindly placed at my 

 disposal by Prof. E. B. Wilson, which I believe were made by 

 him some years back. 



One object I have kept in view has been that of following 

 the changes leading from the gastrula to the formation of 

 the trochophore. In the numerous careful accounts of the 

 development of Annelids that have been published few 

 attempts exist to connect the cell regions of the early stages 

 with the organs of the trochophore. Many of the early 

 embryologists, as Kowalevsky (23), Agassiz (1), Hatschek (18) 

 and Salensky (30) seem to start their studies only with the 

 young larva, when the rudiments of the larval organs have 

 already appeared. On the other hand, many of the more 

 recent investigators, commencing with the unsegmented egg, 

 fi-equently fail to carry their studies far enough when they 

 stop short at the end of gastrulation, and before the definitive 

 organs of the larva have appeared. 



Some considerable confusion has arisen through taking 

 the conditions found in relation with the mesoderm at rela- 

 tively late stages, and considering these same relations to 

 liold in the early phases. This is seen in the work of 

 Hatschek (18) and Fraipont (12) on Polygordius; and has 

 resulted in some error with regard to the head-kidney rudi- 

 ments, larval and coelomesoblast. 



in all Annelids with a free-swimming larva such as that of 

 Eapomatus there is alwa3's a considerable interval between 

 the end of gastrulation and the assumption of the full tro- 

 chophoral condition. This period, for the sake of convenience, 

 I shall refer to as that of the pre-trochophoral stage. It is 

 the period of which we know the least in the development of 

 Annelids. 



The excellent papers of Woltereck (52) on Polygordius 

 and Torrey (41) on Thalassema have done much to advance 

 our knowledge. The early cell-regions have hei-e been traced 

 clearly to their ultimate fate in the organs of the trochophore. 

 Woltereck has shown that the head-kidneys arise early and 



