10 Mr. C. S. Bate on the Morphology of some Amphipoda 



have the basos largely developed, and the remaining joints less 

 so than the normal condition of these appendages generally. 

 The fourth pair of pereiopoda in the young are developed in the 

 character of a very perfect chela, bearing a very similar appear- 

 ance to those of the preceding pair of pereiopoda in the genus 

 Phronima ; but in the adult Brachyscelus they approximate closely 

 to the form and condition of the same organ in the genus Platy- 

 scelus adult, the basos being apparently monstrously developed at 

 the expense of the rest of the appendage. The fifth pair of pereio- 

 poda in the young are not half the length of the two preceding 

 pairs, and, like them, have not the basos enlarged ; the dactylos is 

 represented by an immobile, curved, sharp spine : in the adult 

 the same pair have the basos large, and the remaining joints 

 very short and feeble, while the dactylos consists of a curved 

 hook, apparently immobile. The three anterior pairs of pleo- 

 poda differ in the young from those of the adult by their imma- 

 ture and undeveloped character only ; the three posterior, in 

 the young, have the rami simply styliform, and unequal ; in the 

 adult the antepenultimate only exhibits inequality in the length 

 and the styloid shape of the rami ; the penultimate and ultimate 

 are foliaceous and equal, the ultimate the more perfectly so. In the 

 young the telson is small, whilst in the adult it is as broad at 

 the base as the segment of the pleon immediately preceding, 

 and extends as far as the terminal extremity of the ultimate pair 

 of pleopoda. 



Having noticed the changes which the young of these species 

 pass through previously to attaining their final condition of ma- 

 turity, it will be interesting to observe the relations which they 

 bear to those of other genera and to the Amphipoda in general. 



The great dissimilarity between the form of the adult and the 

 young animal must strike the most casual observer. The great 

 change in the two last-described forms is due to the immensely 

 developed eyes of the adult compared with the almost invisible 

 organs of the young, and to the monstrous growth of the basa 

 of the third and fourth pairs of pereiopoda. The adult form that 

 approximates the nearest to the young of these genera is that of the 

 genus Oxycephalic, which bears so close a resemblance to the 

 young of Platyscelus that they might readily be accepted as be- 

 longing to one genus. In Oxycephalies the cephalon is long, 

 inferiorly concave, tapering anteriorly to a point ; the pereion is 

 not laterally compressed; the pleon has the first segment deeper 

 than the two succeeding ; the telson is long and tapering, and 

 as broad at the base as the preceding segment. According to 

 Milne-Edwards' s figure in his ' Histoire des Crustacea/ pi. 30. 

 fig. 10, and M. Guerin, "The eyes are large; the anterior an- 



