ON OUR PRESENT KNOWLEDGE OF THE CRUSTACEA. 203 



It would therefore necessitate, if the Anomura be excluded from its 

 place in the classification of the Crustacea as a sub-order, as suggested by 

 Claus, that it should go over to the Macrura as a whole, including the 

 genera Dromia and Galathea. But it appears to me that if Claus' figures 

 be those of Albunea (Crustacean Systems, PI. IX., figs. 1-10), as he thinks 

 probable, the evidence is strongly in favour of the retention of the sub- 

 order, for the broad fan-like telson is suggestive of an internal Anomurus 

 structure. 



It would appear, therefore, the evidence is much in favour of the argu- 

 ment that the development of Crustacea shows there is a group of zoaea 

 between the two well-defined orders that exhibit features that belong to 

 it, and are not common to the others ; and these features show an advance 

 in the development of the crustacean embryo before it quits the ovum. 



The nearest form of crustacean life to the zoaea when it quits the ovum, 

 appears in the young of Squilla, which has long been known by the name 

 of Alima. The advancement in development is shown in the distinctly 

 pedunculated character of the eyes ; in the articulated condition of the 

 peduncle and the two distinct branches of the first pair of antennae ; in the 

 character of the gnathopoda, which assumes a resemblance more dis- 

 tinctly typical of the adult feature ; and in the advanced development 

 of the four posterior pairs of pereiopoda. 



Alima is more advanced in development when it quits the egg than 

 zoaea, but not so much so as the young of the genus Homarus, which is 

 developed in what Claus and Fritz Miiller have named the Mysis stage ; 

 that is, the appendages of the cephalon are well advanced towards their 

 adult form, and those of the pereion carry a secondary branch, or ecphysis, 

 attached to the third joint or ischium of each pair of pereiopoda. 



But in this last-named genus we find that some of the pleopoda are 

 present ; and the curious phenomenon exists, that, while an enormous 

 amount of development has gradually proceeded to such an extent that 

 all the appendages are rapidly assuming the permanent type, those of 

 the pleon, which in some genera are in advance of those of the pereion, 

 in Homarus remains in abeyance, and appear not to have progressed 

 beyond the zoasa stage. 



This curious fact is exemplified more decidedly in the genus Palinurus, 

 where the pleon appears to be in a still more embryonic condition, while 

 the cephalon and pereion are distinctly pronounced, in a parallel con- 

 dition with that of Homarus, from which it differs most apparently in the 

 length of the pereiopoda in Palinurus, and in the absence of the chelate 

 character of the first pair of pereiopoda. 



In the young of Crangon boreas (Pbipps) — which naturalists have 

 been divided in opinion as to whether it should be embraced in the same 

 genus with Crangon vulgaris or not — the young is advanced in development 

 beyond the condition of the zoaea as it is in Crangon vulgaris. It 

 quits the ovum with all its appendages conspicuously advanced, whether 

 they belong to the cephalon, pereion, or pleon. This we find to be also 

 the case in the genus Thalascaris, an undescribed deep-sea genus belong- 

 ing to the Challenger collection of Crangonidas. 



A still further advance is found in a form closely allied to Alphceus, 

 that I believe has been recorded as a species, but which I described in 

 the ' Transactions of the Royal Society' under the generic name of Homaral- 

 phmus, on account of its resemblance in the adult form to Alphwus, and 

 in its young to that of Homarus. 



