MUSEUM OF COMPAKATIVE ZOOLOGY. 183 



possessing external hairs ! It is an interesting and at once a significant 

 fact that Chaetosoma possesses a double row of hollow hairs or bristles 

 on a portion of the ventral line. These hairs strongly resemble those of 

 Nectonema, but it is apparent at once from a comparison of internal or- 

 gans that the resemblance is purely superficial, since Chsetosoma is as 

 like the Nematodes s. str. as Nectonema is difi'erent from them ; this is 

 simply an interesting case of the development of like structures in 

 widely different forms, which may be traced perhaps to similarity in 

 their conditions of life. 



In much the same way the resemblance to the Trichotrachelidge 

 emphasized by Burger ('91, p. 649) is at most an instance of the con- 

 vergence of parasitic types. The resemblance is indeed close in the mus- 

 cular and digestive systems. The latter is, however, the system most 

 immediately and directly afiected by parasitism, and such resemblances 

 may easily have arisen independently in any number of animals. The 

 peculiar structure of the oesophagus is shared by the Mermithidge as 

 well ; and so far as the muscles are concerned this type is common to 

 an entire group of Nematodes, the Coelomyaria. On the other hand, the 

 reproductive and nervous systems of Nectonema and the Mermithidse 

 represent opposite extremes in the class Nematoda. 



There is one comparison, however, which deserves more detailed con- 

 sideration. Verrill ('73, p. 632) said of Nectonema, " In general ap- 

 pearance when living and moving, it resembles Gordius"; and again 

 ('79, p. 187) he calls attention to the external similarity of the living 

 animals. Burger ('91, p. 649) enumerates the points of agreement be- 

 tween the two as the absence of lateral lines and the position of the 

 nervous system in the ventral line, and emphasizes the difference in the 

 digestive system and in the structure of the muscles. This is not a suf- 

 ficiently broad and accurate comparison, and it will be valuable to enu- 

 merate here more exactly the points of agreement and difference for the 

 various systems of organs in order. 



The cuticula differs both in thickness and in the possession of rows of 

 bristles and scales in the one form, and of scattered papillae and sensory 

 bristles in the other. The subcuticula has in both the characteristic 

 Nematode nature. The muscular elements show at first sight a con- 

 siderable difference in structure, yet I am convinced that this is more 

 apparent than real. The muscle cells of Nectonema are those of the 

 typical Coelomyarian, in which the muscle fibrillge are arranged in a 

 peripheral fl-shaped layer about the distal edge of the muscle cell. Into 

 the hollow of this contractile portion extends a projection from the 

 plasmatic portion of the cell which is found at the inner border of the 



