METAZOA VERMES. 311 



vided with an embryonic membrane, the amnion, when 

 passing through its development, which is evidence of 

 acceleration in development. The shape of the body of 

 the adult Sipunculus nudus is more nearly equal through- 

 out than in S. tessellatus. The tentacles are clearly seen 

 in the specimen (No. 772), and also the anus near the 

 anterior end. 



NEMATODES. 



Free-living Nematodes are extremely abundant in the 

 sand of beaches and of rivers. One from the Mediter- 

 ranean is represented by Tricoma cincta (PI. 773, figs, i, 

 2, greatly enlarged). While it is true that most Nema- 

 todes are without segments or appendages, yet there is, 

 according to N. A. Cobb, 1 a repetition of the order of 

 arrangement of certain organs, both external and internal. 

 This observer mapped the position of all the hairs on the 

 anterior half of several specimens of a species of Spilo- 

 phora. A comparison of the results showed that the hairs 

 were arranged in almost exactly the same order in each 

 case, and that there was a faint trace of repetition of 

 arrangement such as is commonly observed in segmented 

 worms. Tricoma cincta (PI. 773, A, the head; B, the 

 posterior end of the body) shows external " annulation " 

 which is certainly similar to segmentation, though this 

 does not extend to the internal organs. The body cavity 

 of the Nematodes is hollow and similar to that of Annelida 

 generally. 



Besides the free-moving species there is a great num- 

 ber of parasites. Many of these spend their early larval 

 life in water and afterwards become parasitic for a shorter 

 or longer time as the case may be. 



1 Parasites of Stock, New South Wales Dept. Agric., Misc. Publi- 

 cation 215, 1898, p. 18. 



