DEVELOPMENT OF ANNELIDA. 29? 



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

 Annulosa in general. 



907. Comparatively little is known of the history of the de- 

 velopment of the Annelida ; but there appears to be considerable 

 variation in the amount of change they undergo after coming 

 forth from the egg. The Branchiferous Annelides on first 

 leaving the egg form an oval or roundish body, furnished with 

 one or more bands of cilia, by the agency of which the little 

 creatures swim about freely in the water. The body then gradu- 

 ally becomes more elongated, during which additional bands of 

 cilia often make their appearance. In a little time indications 

 of the eyes and of the division into segments show themselves, 

 a mouth and intestinal canal are formed, and by degrees the cilia 

 disappear, and their place is usually taken by bristles of peculiar 

 construction ; which however are only provisional or larval ap- 

 pendages, and fall off when the worm approaches maturity. 

 The number of segments then gradually increases, the new rings 

 being always produced between the last and the last but one. 

 In this way the body is elongated ; fresh organs are formed on 

 its sides and on the head, until it acquires its permanent form. 

 In the Leech and Earth- Worm, on the other hand, the develop- 

 ment of the young seems to be nearly complete by the time 

 that they leave the egg. We find in this group the first dis- 

 tinct appearance of that gemmiparous mode of reproduction which 

 is especially characteristic of Zoophytes (ANIM. PHYSIOL. 723- 

 730). The accompanying figure represents the mode of pro- 



PIG. 625. SYLLIS PROLIFERA. 



pagation of the Syllis prolifera ; in which a young one is formed 

 from the hinder part of the body of the parent, its head being 



