lxxx 



THE VOYAGE OF H.M.S. CHALLENGER. 



Ill the Dendrobranchiata Lucifer is tbe only genus the development of which has 

 been accurately determined, although the negative evidence arising from the absence of 

 the attachment of ova in all known genera is suggestive of their being fertilised as in 

 Lucifer, and hatched also in the Nauplius stage. 



In the Phyllobranchiata the brephalos quits the ovum as a Zoea, but to this rule 

 there are exceptions, and these may exist in nearly allied species, as in Alpheus 

 and Homaralpheus, which are generically separated on the physiological grounds that 

 Alpheus has the brephalos hatched in the form of a Zoea and Homaralpheus in the 

 form of a Megalopa. Similar reasons suggested the separation of Systellaspis from 

 Acanthephyra and Crangon arctus from Crangon vulgaris. Now if we turn to the 

 genus Oplophorus, which Milne-Edwards has ranged among the Penseidea — chiefly 

 it appears from its having a series of large basecphyses attached to the legs — there is 

 nothing in its general form excepting the non-chelate character of the third pair of 

 pereiopoda which prevents it from being considered a long-spined congener of Sicyonia, 



Fig. XVI. — Oploplwrus lypus, from a drawing by the late Dr. R. von Willemoes Snhm. g\ first gnathopod ; g-, second 

 gnathopod ; p', first pereiopod ; /, basecphyses ; pi', first pleopod. 



which it approximately resembles, yet we know that they differ in the manlier of their 

 development and in the structure of their respiratory organs, and therefore are widely 

 separated in their genealogical history. 



If therefore we utilise our observations on the external form of these recent Crustacea 

 we may be able to read much of their internal structure and organisation, and determine 

 the true relation of the fossil forms to their recent congeners. And I believe that I 

 am near to the truth in asserting that nearly all, if not all, the Macrurous forms that 

 are found in the earliest geological formations belong to the Tricbobranchiata, either 

 Normal or Aberrant. 



There are some genera which have only been deciphered from such very distorted or 

 injured fragments that it is impossible as yet to determine their perfect structure ; such 

 is the case with Palseocrangon (f) socialis, Salter, and of Gilocrangon, Ritchie, of which 



