196 TKE XATl'RAL HISTORY REVIEW. 



the former, or wliat would seem to be sucli, are distinguished from 

 all other Lerneoid Crustacea by their well developed thorax, ex- 

 hibiting distinct traces of five somites. Each is about one-fourth 

 the length of the female, and is furnished with a median rudimentary 

 eye, wanting in the other sex. 



The parasitic relations of this genus are peculiar. In the case 

 of both species only a single female inhabits the same Nudibranch, 

 within the visceral cavity of which she dwells ; the minute extremity 

 of the abdomen and the ovigerous sacs being the only parts pro- 

 jecting beyond the integument of the mollusc selected. S. gracilis 

 " invariably occupies the same position, resting upon the under sur- 

 " face of the hver-mass and embracing two-thirds of it with its long, 

 " attenuated lateral processes." S. Irevipes was always found " im- 

 " mediately below the dorsal skin in the neighbourhood of the heart." 

 Several males of S. gracilis accompanied each female and attached 

 themselves to its body, as in Chondr acanthus, or to the surface of 

 contiguous viscera. Like many other internal parasites they 

 seem to cause little injury to their host, notwithstanding their great 

 comparative size. " >S^. gracilis is not very much shorter than the 

 " length of the liver upon which it lies, and which it almost encircles 

 " with its arm-hke processes ; while S. hrevipes nearly occupies one- 

 " third of the visceral cavity of JDoto coronata, and lives in a position 

 " where it might be thought to interfere with the central organs of 

 " circulation ; and in the case of Holis rufihrancMalis, before alluded 

 "to, the parasite must have been in contact with the cerebral 

 " ganglions. Tet these animals seemed perfectly unconscious of the 

 " presence of the insidious foe that was feeding upon their hfe's 

 " blood. They moved about apparently quite at their ease, and were 

 " in no way distinguishable from unafflicted individuals, except by 

 "the presence of the protruding ovigerous sacs of the parasite. 

 " They had mostly attained their full growth, and there seemed every 

 " probability of their living the usual time allotted to the life of the 

 " species. When they perish, the contained parasite must perish 

 " also ; for it is an inert, helpless creature, quite incapable of any 

 " active exertion in search of food or for self-preservation." 



We commend Dr. M'Intosh's paper to those naturalists who 

 complain of the want of materials for investigation. What can be 

 a more accessible ' subject' than the common shore- crab, yet to a 

 competent and painstaking observer we see how readily it displays 

 new and interesting features. It would not be easy to offer a con- 



