078 ME. W. M. THOMSON ON A BRANCHIATE [Dec. 18, 



supporting the mesial regions of the elytra, a continuous stream of 

 water may, nevertheless, be passed over the back, thus producing 

 the same result as the rhythmical movements observed in Aphrodita. 

 This is interesting as showing how the same need has been met 

 in animals of different structure by adaptations of different kinds. 

 The apparatus in this species seems to have attained a considerable 

 degree of perfection, and in the literature at our disposal I can 

 find no reference to any similar arrangement. 



In this dorsal view further points of interest may be noted. 

 When the elytra were removed in a female, a pale pinkish 

 substance was observed, especially above the bases of the notopodial 

 bunches of hair. This, when examined under the microscope, was 

 found to consist of numerous ova which did not seem to have been 

 fertilized ; for neither were polar bodies to be seen, nor were 

 any eggs segmented, while they all presented the large vesicular 

 nucleus characteristic of the unfertilized ovum. It would seem, 

 therefore, that the ova are all passed up under the elytra (the 

 female contained a great number of eggs in the ccelom), and are 

 there fertilized by sperms carried in by the respiratory current. 

 According to a statement made by Haswell, development must also 

 take place there to a certain extent. 



The elytriferous segments are 2, 4, 5, 7 &c, 21-23, the elytro- 

 phore being a special part of the parapodial ridge. The alternate 

 parapodia carry dorsal cirri with bulbous tips, long enough to 

 protrude freely (as much as 3 mm.) beyond the edges of the 

 elytra. The base of the cirrus is extremely muscular (Plate LX. 

 fig. 3 and Plate LXI. fig. 7), and they are evidently capable of 

 considerable movement. The dorsal cirri of the first, i. e. the 

 peristomial segment, are without this muscular base and resemble 

 the prostomial tentacles in appearance. 



One more point to be noted in this view of the dorsal surface 

 is the anus, which lies just beyond the end of the dorsal respiratory 

 channel, separated from it by the transverse ridge already referred 

 to. The two ventral pygidial cirri represent the ventral cirri of the 

 pygidium, and are directed backwards, as are also the dorsal cirri 

 of three pairs of parapodia immediately anterior to the pygidium. 

 Thus four pairs of these tactile organs may be seen curling up 

 round the edge of the elytra at the extreme posterior end. It 

 seems rather remarkable that there should be such a good supply 

 of " feelers " at this end, and indicates that it is the habit of the 

 animal to explore crannies and holes among stones &c, from 

 which it must often make its exit backwards. 



The Head. — The head consists of prostomium and peristoinium 

 (the first segment), the whole being retractile to a slight degree 

 under the next segment, which bears the first pair of elytra 

 (Plate L?LL. fig. -1). The prostomium is irregularly oval, convex 

 above, well defined posteriorly. Its base is, in the preserved speci- 

 mens, overhung by the first "dorsal tubercle," which belongs to the 

 peristomial segment. There are two pairs of eyes — a posterior 

 larger, and an anterior smaller eye on each side, — which are so 



