41 



of more delicate bristles are placed about the middle of the slightly swollen, anterior margin 

 of each segment. Of those arranged in a transverse row, to begin with the 4"^ or 5"^ segment, 

 two or three are much thicker and stronger and have the shape of claw-like spines. 



The 5th and 6''' cirri are shorter than the foregoing ones, the 6''' being not inconsiderably 

 shorter than the 5"*, the latter equalling about 7/^^ the length of the cirrus of the 4"^ pair. The 

 rami of the 6"^ pair (fig. 8) are slightly unequal and have respectively 7 and 9 segments. 

 On the posterior margin of each segment, near the extremity, a group of spines are planted 

 which correspond with the stronger ones of the transverse rows of spines of the segments of 

 the 2°<^ — 4'^^ cirrus. From the 4''^ to the 8* segment two of these .spines are claw-like and strong, 

 the one, however, more so than the other. The last segment bears two claw-like spines at the tip. 



Caudal appendages (fig. 8) thin, having five tapering segments, and being a little 

 longer than the pedicels of the sixth cirrus. Delicate hairs are planted on the extremity of each 

 segment, the last segment bearing on its summit a tuft of slightly longer hairs, the longest of 

 which is nearly equal to the length of the segment itself. 



Penis (fig-. 8) rather thick, with the terminal part narrow and conical towards the 

 extremity. The middle part is distinctly ringed. A few delicate hairs are scattered over its 

 surface, a tuft of such hairs being planted close by the tip. On each ring of the annulated 

 middle part a transverse row (fig. 8') of short, broad spines of very peculiar shape is planted. 

 They look like little hands with extremely short and pointed fingers. The number of these 

 peculiar appendages is largest on the broadest segments, their maximum number upon one 

 segment being 15. 



The specimens of this species were found attached to the valves, partly to the outer, 

 partly to the inner side of Lithotrya nicobarica. I observed from 2 to 7 specimens — most of 

 them very small ones — attached to one Lithotrya. They must be a nuisance to the animal, 

 especially when they are attached to the inner side of the valves ; they feed, in all probability, 

 on the same planktonic matter which the Lithotrya is trying to get hold of. 



The Lithotrya nicobarica was collected at : 



Stat. 225'. November 8, 1899. 2700 m. N. 185° E. from Southpoint of South Lucipara Island. 

 Reef-exploration. Bottom: stone. 



A small specimen of the same species was found attached to a specimen of Lithotrya 

 conica n. sp. collected during the "Siboga" expedition at '"Kur-Reef", a very small one to 

 one of the specimens of L. truncata, collected at Haingsisi, Samau Island, Timor. 



Observation. I think that this species comes close to Alcpas qtiadrata x\urivillius. 

 I consider and describe it as different, partly because the general shape is quite different, partly 

 because it seems to live under quite different conditions, partly because the structure of the 

 appendages of the animal's body does not agree with that of the species of Aurivillics. The 

 short and straight cirri composed of 8 — 10 segments only are peculiar to the present species. 

 The interesting small spines which cover the surface of the penis occur most probably also in 

 A. qiiadrata. Aurivillius says (1. c. p. 32) that this organ is "hie und da mit rlickwarts 

 gerichteten Borstchen versehen". Perhaps there exists a certain relation between the one or 

 both species and Alepas malaysiana described of late by Ann.\ndale. 



41 



SIBOGA- EXPEDiriE XXXI (7. 



