On the Variations of certain Crustacea. 331 



to protect the respiratory cavity from tlie entrance of the 

 sand in which the creatures live, but a species of Gelasimus, 

 which lives far from the sands, in the mangrove forests, in 

 company with several Grapsidoz, nevertheless has the respiratory 

 orifice concealed between the third and fourth pairs of feet.* 



The limits of this paper do not permit me to follow Dr. 

 Midler in his investigations into the development of various 

 Crustacea. The metamorphoses which these animals undergo 

 are amono-st the most wonderful in the whole rang-e of natural 

 history, fully equalling, and perhaps, from their significance 

 even exceeding in interest, the more generally known changes 

 of the Lepidoptera. The phenomena of embryonic development 

 in all kingdoms of nature lend very strong support to Darwin's 

 theory, and are indeed perfectly inexplicable without recourse 

 to- some such hypothesis, and Dr. Miiller has, by his recent 

 investigations, added considerably to our knowledge of these 

 amongst the Crustacea, and given additional reasons for belief 

 in the theory of transmutation. For the details of his researches 

 I must refer the reader to the translation from which I have 

 already quoted largely. 



The Ostracoda belonging to the families Cyprince and 

 Cytlierince have been hitherto divided into genera and sub- 

 genera on the strength of characters taken in great measure 

 from the carapace, or outer shell of the animal, and more par- 

 ticularly from those of the hinge-joint. This mode of classi- 

 fication must in time give place to one founded on the structure 

 of the animals themselves ; but these are often so difficult to 

 obtain in a living or unmutilated state, and are withal so diffi- 

 cult to examine thoroughly and minutely without the aid of a 

 large supply of specimens, that a thorough knowledge of their 

 organization must involve the labour of a lengthened period. 

 Meantime, a good beginning of the work has been made by 

 Dr. Zenker, Professor Lilljeborg, Gr. 0. Sars, and other natu- 

 ralists. But by far the greater number of Ostracoda hitherto 

 described are fossil species, which can be known only by their 

 shells j or recent species, which have as yet only been examined 

 in a dry state. In these cases the characters of the hinge- 

 joint are very valuable, exhibiting peculiarities which seem to 

 coincide pretty definitely, so far as we at present know, with 

 certain differences of the animal organization. 



When I began the study of the Ostracoda, I supposed that 

 every species (I refer now to those of the families Oytherina3 

 and Gyprinoe, which comprise a vast majority of the British 

 species) might unhesitatingly be placed, by virtue of the pecu- 

 liarities of the hinge and external shell- surface, in one or other 

 of the established genera. What I wish now to show, is that 



* Annals and Magazine of Natural History. Third Series ; vol."xv. 



