176 Royal Society :— 



of forms ; but I doubt if it is so useful as tracing the steps of indi- 

 viduals. ****** 



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

 they are much as the ordinary sea specimens, 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 Henrt Power, 

 " Sfaff-Surgeov, Ufh Regt., Lt. Inf." 



Some time afterwards the author received the promised collec- 

 tion, together with Mr. Power's drawings and notes. These have 

 enabled him to identify the parent forms of some known larvae, 

 and also to determine those of several unknown genera. 



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

 throughout 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 AlpTieus, 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 deve- 

 loped, if not more so, as any of those whose life is passed in the 

 bright sunshine of the surface of the ocean. 



The blind Dldamm brought from the depth of four miles below the 

 surface of the Atlantic by the dredges of the 'Challenger' differs 

 in no respect from Polyclieles, taken by Heller in the comparatively 

 shallow Adriatic sea. In the blind prawn from the Mammoth Cave 

 of America, and the sightless Nqjhrojh'^ 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 

 nny approach to the NaupUus state, as mentioned by Fritz Miiller; 

 so that the author believes that it must be confined to the genus 

 Petieus alone among the Podophthalmia. Xor should it be forgotten 

 that the Navplius form has only been observed as a free-swimming 

 animal. 



The author has taken this opportunity of making a close exami- 

 nation into the eai'lier stages in the development of the embryo, 

 and compai'ing the progress within the ovum of some of the larvae 

 that arrive at or near maturity before being hatched, with those 

 of the larval forms that are hatched in a more immature condi- 

 tion ; and he states that, as soon as the protoplasm assumes any 

 thing like a definite plan, distinct lobes, corresponding in position 

 with those of the several appendages in the Nauplius, together with 

 an embryonic or ocular spot, are present — that in the A^aiy^Zttts forms 

 they exist as deciduous appendages only, and are soon cast aside and 

 replaced by others more adapted to the wants of the adult existence. 



