of the Division Hyperina. 13 



The gnathopoda in the young of Platyscelus possess less of 

 the embryonic condition than those in the young of Vibilia, 

 but the form approximates to that of a mature animal ; they do 

 not resemble those of the parent, but approach more nearly to 

 those of Hyperia or Lestrigonus. The first two pairs of pereio- 

 poda, as in Vibilia and probably all the genera of the family, 

 assume in the young the form of simple poda, and differ from 

 those of their respective parents only in the relative proportion 

 of parts and in the presence or absence of a few spines or teeth. 

 Not so the third and fourth pairs. In the three genera to 

 which this paper alludes, these assume a very different form in 

 the young from that presented by the adult : in Platyscelus and 

 Brachyscelus they exhibit a condition that would generally be 

 accepted as more perfect than that of the adult ; whilst in Vi- 

 bilia, although the change is quite as great as in the preceding 

 genera, their form in the parent appears adapted to fulfil similar 

 conditions to those of that in the young, being apparently as com- 

 plete for the purpose in the one as in the other, although bearing- 

 little or no resemblance. In Platyscelus and Brachyscelus 

 their form and condition are altogether distinct, and must be 

 different in kind as well as in degree. The fifth pair of pereio- 

 poda, in the genera under consideration, exhibit altogether de- 

 pauperized features ; and it is curious to observe, with this 

 condition constant in the adult state, how variable are their 

 forms in the young animals of the several genera. In Vibilia 

 they are, in the adult, well formed, but slender and feeble; in 

 the young they are robust, but embryonic in appearance. In 

 Platyscelus they are rudimentary in the adult, but in the young, 

 with the exception of the dactylos, they are as well developed 

 as the preceding pair, and only differ from them in having a nar- 

 rower basos ; and in Brachyscelus, where they approximate some- 

 what to the form of the preceding pair in the adult, in the young 

 they assume a somewhat abnormal condition. Thus, singular to 

 say, in that genus in which the appendages are most rudimentary 

 in the adult, they are most perfectly developed in the young ; and, 

 on the other hand, in that genus in which their character in the 

 adult is most consistent with that of the preceding pairs of appen- 

 dages, they are, in the young, the most aberrant and immature. 



In judging of the changes that these animals undergo, they 

 appear to consist in something more than simple development. 

 The morphology of some of the parts is certainly as complete a 

 change of one form to another as is conceivable. Admitting that 

 the change of the cephalon from the long, flat, tapering process to 

 the short globular lobe is but the result of the development of 

 the eyes and the consequent growth or enlargement of one part 

 at the expense of another, it cannot be the same with regard 



