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State what you know about the Reproductive System of 



Arenicola. 



The sexes are separate. The reproductive elements or 

 gametes arise from patches of the coelomic epithelium 

 which covers the nephridia. (There are six pairs of neph- 

 ridia in the anterior region of the body.) The gametes are 

 liberated in the body cavity, and pass through the nephridia 

 to the outside. 



Reproduction and Development of Polychaeta. 



Write an Account of the Reproduction and Development of 

 Polychaetes. 



The sexes are generally separate and the sex cells or 

 gametes are always formed from the coelomic epithelium. 

 ' The development is indirect ; the segmentation of the egg 

 results in a pelagic free-swimming larva of the trocho- 

 sphere type. This trochosphere has the appearance of two 

 small cones attached together at their bases ; there is a 

 single or double girdle of cilia around the middle, the 

 mouth is situated at the girdle, and the anus is at the apex 

 of the lower cone which lengthens out and becomes seg- 

 mented as the body of the future worm. The upper part 

 of the trochosphere becomes the head. 



In certain species of the genus Nereis (when sexually 

 mature) the posterior (sexual) portion of the body is 

 strikingly different from the unchanged anterior (non- 

 sexual) portion ; the parapodia having been transformed 

 into leaf-like swimming appendages. This is called the 

 41 Heteronereid " phase. 



In many instances (e.g., Nereis dumerilii) there are also 

 different forms of both sexes (polymorphism). 



In the family Syllidae similar reproductive phenomena 

 occur ; but there is also asexual reproduction by fission 

 and budding, a sexless worm producing one or more worms 

 of one sex (i.e., schizogamy, showing alternation of genera- 

 tions). Illustrative of this are the following : 



Syllis hyalina. The posterior sexual half of the " heterosyllid " 

 separates by fission from the anterior non- 

 sexual half, and forms a new head. 



