DEVELOPMENT OF ANNELIDA. 301 



greenish tint ; and in many instances it is colourless, as in the 

 Annelida in general. 



836. Little is known of the history of the development of the 

 Annelida ; but there appears to be considerable variation in the 

 amount of change they undergo after coming forth from the egg. 

 The Nereidans, in their early stage, seem to bear a very close 

 resemblance to Infusory Animalcules ; for they are described as 

 consisting of a simple disc, surrounded with two rows of vibratile 

 cilia, which serve for locomotion, and for the acquirement of 

 food. The mouth, which is situated near the edge of the disc, leads 

 to a short, straight alimentary tube, which terminates in a sort of 

 cone projecting from its lower side. In a short time this conical 

 projection begins to increase in length, and to show a division 

 into segments ; and by a gradual increase in the number of these, 

 the body is at last formed. At the same time, the formation of 

 the head takes place by a gradually-increasing projection of the 

 other surface of the disc. Whilst the head and body are being 

 developed, this first-formed disc remains for a time as a sort of 

 appendage on each side of the head ; but it finally disappears. In 

 the Leech and Earth- Worm, on the other hand, the development 

 of the young seems to be nearly complete, by the time that they 

 leave the egg. We find in this group the first appearance of that 

 gemmiparous mode of reproduction, which is especially charac- 

 teristic of Zoophytes (ANIM. PHYSIOL. 723-730). The 

 accompanying figure represents the mode of propagation of the 



FIG. 524. NEREIS PROLIFERA. 



Nereis prolifera ; in which a young one is formed from the hinder 

 part of the body of the parent, its head being produced by a 

 conversion of one of the segments of its body ; and this is gra- 



