and the Embryology of Autolytus cornutus. 403 



through wliich the embryo Autolytus passes will give us 

 valuable hints for the classification of Annelids. All the 

 earlier stages, before the rings can be seen, have a very 

 striking resemblance to the Planarians, and similar worms- 

 They are flat, the eyes are simple accumulations of pigment 

 cells, there are no divisions into rings perceptible, and the 

 principal feature is the great development of the alimen- 

 tary canal. In the next stage they become more elongated, 

 the rings are strongly marked, there are rudimentary 

 bristles, or gills in the case of the embryo of Leucodore and 

 Hcematorhea ; and in this more advanced stage they recall 

 to us the Nematodes^ or round worms. In the still more 

 advanced stages, the tentacles of the head, the division of 

 the alimentary tube into distinct regions, the presence of 

 well-developed cirri and setse, represent the true Annelids ; 

 thus plainly placing, upon embryological data, the Plan- 

 arians and the flat worms (Platodes), in which the great 

 preponderance of the alimentary tube is the main feature, 

 lowest; then the round worms (Nematodes), in which we 

 have rudimentary gills, etc., next in rank ; and ending with 

 the true Annelids, in which we have the greatest speciali- 

 zation of the appendages of different rings. The number of 

 worms whose embryology is well known is so small, that 

 it is impossible to collect sufficient data for tracing out this 

 correspondence as minutely as could be desired. It is appar- 

 ent from these observations upon the alternate generation 

 of the higher Annelids, that henceforth an accurate knowl- 

 edge of their complete development will be necessary before 

 we can arrive at any satisfactory conclusions with respect 

 to their classification ; that, since we know that species 

 belonging to genera hitherto placed in different families 

 are only different generations of one and the same species, 

 we should be particularly cautious in characterizing gen- 

 era however much they may apparently differ from one 

 another. 



