352 H. s. PRATT, 



among the Copepods and other small animals composing the tow. 

 It was first noticed about the middle of July, although the tow had 

 been examined almost daily since the fifth of the mouth, and was for 

 some time very rare, but one or two specimens being found in the 

 tow collected in any one day. Towards the end of the month, however, 

 it became more common and by the first week in August, by diligent 

 search, several specimens could be found in every haul. 



The first ones observed were all moving about freely in the sea- 

 water. About a week after the first worm was seen, however, it was 

 also found in the body-cavity of one of the common Copepods of the 

 place (the name of which I was not able to ascertain), and from the 

 beginning of August until the 14**" of the month, when I left Cold 

 Spring Harbor, it occurred about as frequently in these Copepods 

 as in the sea-water. At that time it had become fairly common in 

 both places, so that in every watch-crystal full of tow I could find 

 several specimens both within the Copepods and free-swimming. 



The characteristic anatomical feature of appendiculate Distomes 

 is the appendix, the caudal, protrusible portion of the body which 

 can be drawn in and out. Those Distomes which possess this pecul- 

 iarity were placed in a sub-genus of the genus Distoma by Dujakdin 

 (1845^ p. 383, 389, 420) which was named h j that author, ApoUema: 

 and two years later this sub-genus was raised to the dignity of a 

 genus by Blanchard (1847, p. 302). Juel (1890), not knowing of 

 Blanchard's paper, independently gave the group generic value, and 

 MoNTiCELLi (1891) and Braun (1892, p. 570), Avho also do not men- 

 tion Blanchard, follow Juel and accept the genus as valid. It has 

 since been accepted by Stossich (1892), Looss (1896), Raillet (1896) 

 and Stiles (1898). Monticelli (1. c.) has divided the genus into 

 two sub-groups based on the length of the appendix : in the members 

 of one group the appendix is very short, being in some cases not 

 more than a fortieth of the length of the rest of the body; in those 

 of the second group it is very long and may equal or exceed the 

 length of the body. Apoblema ocreatum Olsson may be taken as a 

 type of the first group, and Apohlema apj)endiculaium Rud. as a 

 type of the second. 



The worm in question falls in the latter group; its appendix, 

 when extended, is about one third of the length of the trunk (PL 25, 

 Fig. 3). In the smaller individuals I observed, and in all those oc- 

 curring in Copepods, the appendix was invariably withdrawn within 

 the body, where it formed a thick-walled vesicle communicating with 



