PROGRESS OF MICROSCOPICAL SCIENCE. 
263 
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 specimens, and they certainly 
spend their larval life in the sea. 
" Yours very truly, 
" WiLMOT Henry Power, 
" Staff-Surgeon, Uth Eegt., Lt. Lifr 
Some time afterwards the author received the promised collection, 
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 sup- 
pressed or rendered abortive for want of use are never absolutely lost, 
and may be reproduced under conditions that may require them. 
The eyes of ihose Crustacea, such as Alphmus, 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 sun- 
shine 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 PolycheJes, taken by Heller in the comparatively 
shallow Adriatic sea. In the blind prawn from the Mammoth Cave 
of America, and the sightless Nephrons of Formosa, the organs of 
vision are reduced to the smallest condition consistent with tlicir 
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-swimmiug animal. 
The author has taken this opportunity of making a close examina- 
tion into the earlier stages in the development of the embryo, and 
comparing the progress within the ovum of some of the larvj« tliat 
arrive at or near maturity before being hatched, with those of the 
larval forms that are hatched in a more immature condition ; and ho 
states that, as soon as the protoplasm assumes anything like a definite 
plan, distinct lobes, corres])onding in position with those of the several 
appendages in the Nauplius, together with an embryonic or ocular 
spot, are present ; that in the Nauplius 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. 
In the embryos of other Crustacea the anterior pair of lobes 
enlarge in size with little alteration of form, while the posterior two 
