REPORT ON THE CRUSTACEA MACRURA. xliii 



of the Dendrobranchiata does not appear to take place in this pair, but rather in the 

 penultimate pair, as in Pasiphsea (PI. CXL. fig. In). 



The First Pleopoda. — The first pair of pleopoda is an important pair of organs, deviating 

 from the normal form seen in those posterior to it, and being utilised to assist the male 

 to a greater or less extent in the act of copulation. 



It frequently varies very considerably in form in the two sexes, but it is most 

 pronounced in the males, in which, however, it shows considerable differences in form in 

 different genera and families, but more decidedly in the several divisions into which the 

 Macrura are divided. 



The normal condition of the appendage is that of two foliaceous branches attached to 

 the extremity of a basal joint, which is articulated to the inner wall of the plates that 

 project on each side of the pleonic somites ; these, so far as 1 know, have never 

 received a special name, but are generally considered to be the lateral projections of the 

 several somites. A similar condition of structure exists in the Edriophthalma, and since 

 in these there is no carapace covering the pereion and shielding the branchiae from 

 accident, the first joint of the legs is produced in a similarly squamose manner, over- 

 lapping and protecting the branchiae, situated pendent on the inner side. 



I believe that this is precisely the condition of the great lateral plates on the pleon in 

 the Macrura ; an idea that I have long entertained, and which, I believe, is capable of 

 demonstration by well-grounded arguments. 



That the two rami are homotypical of the exopodite and the endopodite of the 

 pereionic limb, as described by Milne-Edwards — or, as I have preferred to call them, of 

 the pereiopod and its basecphysis (or in plain English the leg with the branch of its 

 second joint) is, I believe, universally accepted as theoretically true ; but the large over- 

 lapping lateral plate is figured and described as part of the somite by Milne-Edwards in 

 his great work Histoire naturelle des Crustaces, while it is omitted in his later memoir 

 on the morphology of the Decapod Crustacea. 1 



In the different genera the second or basisal joint of the appendage is seen to creep 

 (as it were) down the side of the large scale often nearly as far as its lower margin ; this 

 is apparent in several species of Alpheus where it is largely developed, especially in the 

 females as a protection for the ova. In some specimens of an undescribed genus 

 recently taken in the " Talisman" by M. A. Milne-Edwards these lateral plates are so 

 largely developed in the females that they wrap over and cover the ova as in a marsupial 

 pouch. In other genera, as Sergestes and Lucifer, they are reduced to a minimum, having 

 the pleopoda articulating at the extreme margin of the lateral wall. This we might 

 suppose to be liable to occur in such genera as these, in neither of which are ova ever 



1 Observations sur le squelette tegunientaire des Crustaces decapodes et sur la niorphologie de ces animaux, Ann. d. 

 Set. Nat., ser. 3, t. ivi. pp. 221-291, pis. 8-11, 1851. 



