1876.] Development of the Crustacean Embryo. 377 



from the egg, canie under notice only once ; ' again, ' This larva (taken 

 on the 13th of January) is closely approached by four others, probably 

 helonging to the same swarm, which were taken at the same time (24th 

 January) ; ' and so on. 



" To tow a net in these tropical seas and to examine all the micro- 

 scopic Crustacea would give a most extraordinary assemblage of forms ; 

 but I doubt if it is so useful as tracing the steps of individuals. 



******** 



" I have not yet hatched the land Hermit-crabs, though I suppose 

 they are much as the ordinary sea specimeus, and they certainly spend 

 their larval life in the sea. 



" Pray excuse my rambling letter, and please let me know of any way 

 in which I can be of any use to you in my humble dips into natural 

 history. 



" Tours very truly, 



" Wilmot Hekry Power, 



" Staff- Surgeon, Ufh JRegL, Lt. Inf. 1 ' 



Some time afterwards the author received the promised collection, to- 

 gether with Mr. Power's drawings and notes. These have enabled him 

 to identify the parent forms of some known larva?, and also to deter- 

 mine those of several unknown genera. 



It has also led him to the conviction of a unity of character through- 

 out the various forms and changes of Crustacea • that variety in form 

 is never inconsistent with homological truth ; that parts suppressed or 

 rendered abortive for want of use are never absolutely lost, and may be 

 reproduced under conditions that may require them. 



The eyes of those Crustacea, such as Alphceus, that inhabit dark places 

 are reduced in power according to the condition of their habitat. But these 

 organs are, in their larval state, as well developed, if not more so, as 

 any of those whose life is passed in the bright sunshine of the surface of 

 the ocean. 



The blind Didamia brought from the depth of four miles below the surface 

 of the Atlantic by the dredges of the 'Challenger' differs in no respect 

 from Poh/cheles, taken by Heller in the comparatively shallow Adriatic sea. 

 In the blind prawn from the Mammoth Cave of America, and the sight- 

 less Nephrons of Formosa, the organs of vision are reduced to the smallest 

 condition consistent with their retention ; and in the Cirripedes the 

 eyes are represented by their nervous apparatus only. 



The several forms of larva have not, in the prawn-allies, shown any 

 approach to the Nauplius state, as mentioned by Fritz Miiller, so that 

 the author believes that it must be confined to the genus Penceus alone 

 among the Podophthalmia. Nor should it be forgotten that the Nauplius 

 form has only been observed as a free-swimming animal. 



The author has taken this opportunity of making a close examination 



TOE. XXIV. 2 E 



