MUSEUM OF COMPARATIVE ZOOLOGY. 183 
possessing external hairs! It is an interesting and at once a significant 
fact that Chetosoma 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 Cheetosoma is as 
like the Nematodes s. str. as Nectonema is different 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 Trichotrachelide 
emphasized by Biirger (’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 affected by parasitism, and such resemblances 
may easily have arisen independently in any number of animals. The 
peculiar structure of the cesophagus is shared by the Mermithide as 
well ; and so far as the muscles are concerned this type is common to 
an entire group of Nematodes, the Celomyaria. On the other hand, the 
reproductive and nervous systems of Nectonema and the Mermithide 
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. Birger (’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 Celomyarian, in which the muscle fibrille are arranged in a 
peripheral N-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 
