228 Analyses of Boohs. 



Lepades. 2. That they have no relation whatever with the Testacea, 

 as supposed by Linnaeus and the older systematists. 3. That the 

 cru.stacea, now, therefore, furnish examples of a class in which we 

 have animals free and fixed, with eyes and eyeless, and with the 

 sexes separated in some and united in others, all of which are 

 characters, to which, naturalists have attached the greatest impor- 

 tance, as regards classification. 4. That the proof of metamorphoses 

 being fully and satisfactorily established, tends still to maintain the 

 affinity so long recognized between the Crustacea and insect a. 



His description of the change in the Cancer msenas is striking, and 

 deserves to be noted, as it details another change which crabs undergo, 

 and cancels another genus in natural history. He had formerly 

 found, that the young of the common market crab (cancer pagurus) 

 first presents itself as a zoe, and that a full grown zoe passed into 

 some other more perfect form. He has since ascertained that this 

 state must have been that of a megalope. He ascertained this fact 

 by keeping a number of individuals of a megalope in regularly re- 

 newed sea water. These began after a short time to change into a 

 minute crab, until the whole of them, about two dozen in number, 

 were so changed. The Cancer Manias, he finds in its first or zoe 

 stage, is wholly natatory from structure, while in its second it 

 occasionally walks by means of its thoracic members, now become 

 simple ; but more commonly swims by the motion of its sub-abdominal 

 fins, which are greatly developed for this purpose. In both stages it 

 is, therefore, a macroura, but only in the latter, evidently, related 

 to the decapuda. 



Mr. Thompson concludes his papers by noticing the indifference of 

 our zoologists to these important discoveries, and the activity of our 

 French neighbours in respect to the subject. The Institute deputed 

 two naturalists to spend a summer at Isle Re to make observations. 

 The report of Milne Edwards, one of them, was unfavourable to the 

 statements of Mr. Thompson. He pronounced that the Crustacea 

 were hatched with the form and structure of their adult parent. Is 

 the matter to rest here ? We can discover no reason why it should. 

 Mr. Thompson, we believe, to be an accurate observer. We believe 

 Mr Edwards to be the same ; but there cannot be a doubt, that the 

 former has possessed infinitely better opportunities for scrutinizing 

 the phenomena in question, than the latter, or, perhaps, than any 

 other naturalist. The satisfactory mode of deciding the matter would 

 be, that Mr. Thompson should exhibit facts similar to those he has 

 described to the most competent persons for appreciating them, and 

 we have no doubt that some of our naturalists, in the course of the 

 ensuing summer, wdl visit the scene of Mr. Thompson's labours 

 (Cork) with this object in view. 



Remarks on the difficulty of distinguishing certain genera of 

 Testaceous Mollusca by their shells alone, and the anomalies in 

 regard to habitation observed in certain species. By John Edward 

 Gray, F. R. S., &c. — In this paper the subject is considered under 

 two views. 1. In reference to shells apparently similar, but be- 



