558 CEKSSWELL SHEARER. 



find a very large number of errant forms, which have under- 

 gone so much modification that in many cases it is difficult 

 to say to which group they belong. In the first class of the 

 pelagic type we have the larvae of Nereis, Phyllodoce, 

 and Aphrodite, while in the second we have forms like 

 Diopatra, Ophryotrocha, and many of the Eunicidlarvte. 

 Probably the most difficult to class of all are these last, on 

 account of their great variation (Hiiclcer, 13). 



AVithout some knowledge, therefore, of the mode of life of 

 the primitive ancestors of the Polychaets, and the conditions 

 under which they existed, it is difficult to decide which of 

 these various larval types is the most primitive. It is ])ro- 

 bable that the free-swimming type has been recently evolved, 

 and is a more highly modified one than the uniformly ciliated 

 type, that the trochal has been derived from the atrochal 

 form. And this is borne out by the fact that in its most 

 perfect form, as in P]upomatns, it is found in so relatively 

 few Annelids. 



2. Rkview of Literature. 



The early development of the Serpulid Annelids has been 

 studied by a number of investigators. The earliest account 

 is that of Stossich (39) in 1878, who described in some detail 

 the development of fSerpula uncinatus (Eupomatus) and 

 S. glomerata. It is clear from his figures that many of 

 his larva3 were abnormal. I have obtained many similar 

 larvee during the hot months in Naples, when the temperature 

 of the Laboratory sea-water was unusually high. Through 

 the study of these larvse Stossich came to many erroneous 

 conclusions. 



Salensky (30), in 1883, studied the developmentof Psygmo- 

 branchus and Terebella. In these forms the presence of 

 a considerable quantity of yolk and the absence of a true 

 ti'ochophoral stage considerably modify the course of deve- 

 lopment, lie arrived at no certain conclusions regarding 

 the origin of the mesoderm, although he observed the teleo- 

 blast cells of the mesoderm bands. 



