288 ME. GEORGE M. THOMSON ON A FEESHWATER 



peduncle is 3-jointecl, with the joints broad and somewhat flattened ; the first and second 

 joints are subequal in length, the basal slightly the largest and widest, and having a 

 longitudinal suture tending somewhat obliquely outward along its upper surface ; the 

 second joint also bears a slight longitudinal groove on its upper surface, and is fringed on 

 its outer margin with plumose setae. The third joint, w^hich is also similarly fringed on 

 its outer margin, is broader at its distal than its proximal end, and carries two whip-like 

 flagella. The outer flagellum is from two-thirds to three-fouvths the length of the body, 

 and is divided into very numerous articulations. In the yovmgest specimen examined, 

 the body of which was only about 9 mm. long, there were 36 articulations on the outer 

 and only 8 on the inner flagellum; but in adult specimens the number varies from 

 70 up to 92, Avhich was the largest number observed. The articulations are furnished 

 with a few short spinose setse on both sides. On the ten or twelve joints nearest the 

 peduncle the setse are long and plumose, but they tend to become shorter and more 

 simple on the joints nearer the extremity. On all the joints but those few setose ones 

 nearest the peduncle there are blunt hairs or cilia of a very characteristic appearance, 

 which are probably sensory organs. 



The inner flagellum is about one-fourth the length of the outer, and usually consists 

 of about one-fourth the number of articulations ; the joints nearest the base have more 

 or less plumose setse, those further out have the setse simple. But in the male this 

 inner flagellum is modified in a curious manner (PI. XXVI. fig. 12). The fifth and 

 sixth joints are much dilated ; the former bears a large tuft of slightly phimose setse, 

 while the latter cai'ries two large stout setse, w^hich are rugose in their distal half; each 

 is also apparently furnished with a nerve-fibre. The first seven joints also bear a 

 number of short, curved, finger-like setse, all more or less covered with rugosities, so as to 

 resemble a round rasp or file ; these rugosities are arranged in an annular manner and 

 appear to consist of minute stiff hairs. These thickened processes are scattered 

 somewhat irregularly, there being a tuft of about six or seven on the basal joint, then 

 isolated ones or in twos or threes on the succeeding joints, about seven on the sixth 

 joint, and three on the seventh; they are directed toward the upper surface of the 

 flagellum, w^hile the long setse of the proximal joints are all on the lower surface. The 

 remaining joints are simple, and carry short simple setse as in the corresponding organ in 

 the female. These peculiar processes in the male are probably sensory in their function ; 

 they certainly form a very distinct sexual modification. 



The auditory cavity is placed in the basal joint of the jieduncle of the first antennae, 

 its roof being formed by the flattened upper surface. It communicates with the water 

 outside by a duct which opens at the extremity of the joint under a fringe of setse. 



Second antennce (PI. XXIV. fig. 5). — The peduncle consists of four (perhaps five) joints, 

 and bears a long flexible flagellum ; all the joints are more or less flattened out laterally, 

 but the upperside of the first and second joints is produced into a longitudinal crest or 

 ridge, which gives the base a somewhat triangular section. The first and second joints are 

 short and subequal in length ; both bear on their outer extremities a crovm of short spines, 

 and the second joint has also a tuft of fine setse on its inner extremity. This joint also 

 bears an oval plate (the scaphocerite of Spence Bate) fringed with long plumose setse. 



