FRESH-WATER NEREIDS FROM THE PACIFIC COAST AND HAWAII. 213 



articulated, the dorsal posterior ones having five joints and the longer pair of anterior 

 ones the same number. 



The pharynx is retracted. In the specimen mounted in balsam no paragnaths 

 can be seen, but the jaws are visible, and are seen to have very few teeth not over 

 three or four. 



The parapodia (PI. XVII, Fig. 25) are similar to those of Lycastis hawaiiensis, 

 but have a dorsal cirrus that, compared with the same organ of L. hawaiiensis, is 

 proportionately longer on the anterior somites, shorter on the posterior ones, slen- 

 derer, and terete throughout the series. There are fewer but stouter setae, 5 to 7 

 in number. They are unusually large (PI. XVI, Figs. 14-16) for so small a species, 

 but otherwise present no unusual character. 



The specimen is small, measuring only 15 millimetres in length and 1 millimetre 

 in breadth. The number of somites is 55. 



In this species, as well as in L. hawaiiensis, the tip of the foot can be retracted. 

 In these preserved specimens most of the tips are protruded, and practically all are 

 in L. hawaiiensis. The setse seem to be always inserted into this retractile portion, 

 and the cone of retractor muscle-fibres is clearly seen both in optical and in actual 

 sections (PL XVII, Figs. 21, 26). 



Lycastoides gen. nov. Prostomium small, bilobed anteriorly, the lobes pro- 

 duced to form antennae; proboscis without paragnaths; no eyes; parapodia without 

 ventral ligules or dorsal rami, tip retractile. 



A study of sections reveals in all three species a remarkable thinness of the body- 

 wall, and evident weakness of the musculature. The vascular system, on the other 

 hand, has acquired a very high development. While not a few of the smaller and 

 strictly marine nereids have body walls so thin and translucent that the blood- 

 vessels shine through with wonderful distinctness, the muscular development in 

 proportion to the size of the worm is rarely so slight as in these fresh-water forms. 

 For comparison I have chosen Nereis agassizi Ehlers, a North Pacific species that 

 bears a fair degree of resemblance to N. limnicola both as to size and general aspect. 

 The striking difference in the longitudinal musculature of the two forms is best appre- 

 ciated by comparing transverse sections (PI. XVII, Figs. 28, 29). In N. limnicola 

 the circular musculature is so thin that it is practically a mere line except in certain 

 places where it thickens somewhat; but on the other hand it is not much thicker 

 in N. agassizi, while there is great disparity in the development of the longi- 

 tudinal muscles. 



As regards the longitudinal musculature, much the same condition obtains in 

 Lycastis hawaiiensis and in Lycastoides alticola as in Nereis limnicola. In the species 



