REPRODUCTION AND DEVELOPMENT 



419 



which this is accomplished is not very clear, but the cilia on the 

 funnel-shaped internal opening of the tube seem to have some 

 power of selecting the generative cells when they come within 

 their reach, and of passing them on, whilst they reject the much 

 smaller corpuscles of the perivisceral fluid, which are never found 

 in the nephridia. 1 Once inside the internal opening, the clusters 

 break up and the spermatozoa escape singly into the sea. Here 

 they meet with and fertilise the eggs which have escaped from 

 the body of the female. 



Development. Hatschek, 2 who investigated the develop- 

 ment of Sipunculus nudus at Pantano, an inlet of the sea 

 near Messina, states that the 

 spawning takes place during the 

 night, and ceases about July 

 10. The rate of development 

 depends upon the temperature, 

 but the larvae usually free them- 

 selves from the egg -membrane 

 during the third day. When 

 hatched the embryos lengthen 

 out a good deal, and take the 

 form represented in Fig. 213. 

 The larva swims actively by 

 means of a ring of stout cilia, 

 which encircle the body just 

 behind the mouth. Other shorter 

 cilia are found on the head, con- 

 tinuing into the lining of the 

 mouth, and a little bunch of 

 them is situated at the extreme Flo 2 i 3 .-Larva of Sipunculus nudu, L. 

 posterior end. The alimentary 

 canal is already formed, and is 

 twisted, so that the anus lies 

 dorsally, but not so far for- 

 ward as it does in the adult. A 



glandular structure opens into the mouth, and another body of 

 unknown function is connected with the oesophagus ; both these 

 disappear during larval life. A pair of excretory tubules, the 



1 Conn, Stud. Johns Hopkins Univ. vol. iii. 1884-87, p. 351. 

 2 Arb. Instit. Wi.cn t Bd. v. 1884, p. 61. 



x!50. (After Hatschek. ) a, Mouth ; 

 b, anus ; c, excretory organ ; d, glandu- 

 lar appendage of oesophagus ; e, wall 

 of stomach over which the retractor 

 muscle runs ; /, invaginated sense- 

 organ at aboral pole. 



