556 CRESSAA^ELL 8HEAEEK. 



of the Serpulids. Out' text-books frequently cite it as a 

 typical trochopliore, altlioiigli most Annelids possess a 

 ti'ocliopliore quite different. The trochopliore characteristic 

 of the majority of Polychtets is one such as that of Sabella 

 or Nereis^ and not that of the Serpulids. This possesses no 

 liead-kidney, and the mesoderm bands develop under con- 

 ditions that modify their growth as compared with those of 

 Eupomatus. The blastocoelic cavity in these is always 

 greatly reduced or entirely obliterated, and gastrulation is 

 usually epibolic ; while in the Serpulid larvfe there is always 

 :i, large blastocoelic cavity, and gastrulation is by invagina- 

 tion. The egg in the majority of the Serpulids, again, is 

 small and contains very little yolk, although forms like 

 Spirorbis and Sabella contain a considerable quantity. 

 It is hard to make any fast distinctions, however, for larvae 

 occur in the same family, and even in the same genus, which 

 differ entirely in this respect. The principal cause of this 

 great diversity of form is due in most cases to the modification 

 undergone by their locomotor organs, as the I'esult of their 

 adoption of different life-habits. Frequentl}^ closely related 

 larval forms differ greatly in this respect. If they live a 

 free-swimming pelagic existence, or the contrary, their loco- 

 motor organs are correspondingly developed or reduced. 

 Terebella conchilega, leading a pelagic life, possesses 

 strongly developed ciliated rings, and is a powerful swimmer, 

 while Terebella m eckelii, for the most part spending its 

 larval existence in the jelly-like mass in which the eggs are 

 deposited, is uniformly ciliated, and lacks these structures. 



Thus the tuberculous PolycliEets can be divided roughly 

 into two classes on the basis of their possession or non- 

 possession of a trochoplioral stage. The first of these, 

 including Eupomatus, Pomatoceros, and Psygmo- 

 liranchus, possess typical fi-ee-s\vimming larva? with well- 

 developed pi-ototroch and ciliated rings; while a second 

 group, including some of the Terebellids, Aricia and 

 Ar(;nicola, do not possess a free-swimming stage, aie often 

 uniformly ciliated, and are poor swimmers. In addition, we 



