TRANSACTIONS OF SECTION D. 753 



him in his most valuable speculations. In one matter, however, I would venture to 

 express a difference of opinion. He regards the littoral forms of invertebrates as 

 migrating into the deep sea by the following process : Their free-swimming larvae 

 are supposed to be carried out by currents far from land, and then, having com- 

 pleted their development, to sink to the bottom, where a very few survive and 

 thrive. It is hardly to be conceived that any animal, especially in a young and 

 tender condition, could suddenly adapt itself to the vast change of conditions 

 entailed in a move from littoral to deep-sea life. It seems to me much more 

 likely that the move of animals from the shallow to the deep sea has been of the 

 most gradual kind and spread over long series of generations, which may have 

 migrated downwards, perhaps a fathom or so in a century, partly by very slight 

 migrations of the adults, partly by very short excursions of larvae. Thus alone 

 by almost insensible steps could animals, such as those under consideration, be 

 enabled to survive an entire change of food, light, temperature, and surroundino-s. 



The following Papers were read :— 



1. On the Geographical Distribution of the Macrurous Crustacea. 

 By C. Spence Bate, F.B.S. 



The Macrura may conveniently be classified into three natural groups of 

 apparently equal importance in size and structural character. These differ from 

 «ach other in anatomical details, even where they approximate in external form ; 

 and they are essentially distinct in consanguinity by a large history in their develop- 

 ment and growth. 



The distinction is more conveniently exhibited in the structure of the branchial 

 apparatus, which differs importantly, and assumes three different forms. 



In the Astacidea — the lobster and crayfish division — the branchiae are formed 

 as a mass of cylindrical tubes varying in length, but thickly grouped together in 

 a greater or less number of rows opening from one central stalk. 



In the Penreidea the branchiae consist of filaments dividing into branches, 

 sometimes dichotomously and sometimes unequally. These filaments are generally 

 cylindrical in form, but in some genera they are so closely arranged that they 

 become compressed into plates of considerable tenuity, still retaining, however, 

 their divided or branching character. 



In the Caridea— the prawn and shrimp section — the branchiae are in the form 

 of broad plates of extreme tenuity, arranged similarly to those that exist in crabs 

 or the short-tailed Crustacea, the entire plume being suspended by the centre to 

 that portion of the animal to which it belongs. 



With each of these there is a corresponding condition of the walking legs. 



In the Astacidea all have a tendency to possess a chelate condition, of which 

 the first pair is the largest and tbe third the smallest, while the posterior is 

 usually chelate only in the females. 



A departure from this exists in those genera that belong to the aberrant family 

 of Stenopidce, which possess the branchia of the Astacidea, the pereiopoda of the 

 Ten&idea, and the development of the Caridea. 



In the Venceidea the three anterior pairs of pereiopoda only are chelate, and this 

 feature increases in importance posteriorly, and in the depauperised species the first 

 and second pairs depart from that condition, whereas the third, however feeble, still 

 retains the chelate character ; the fourth and fifth pairs are never chelate, gener- 

 ally enfeebled, rudimentary, or obsolete. 



In the Caridea there are never more than two pairs of legs chelate, and the 

 posterior three pairs are invariably simple and, generally, robust and efficient organs 

 of a pediform character. 



With each of these three divisions there is more or less constant condition of 

 development. 



In the Astacidea the embryo always leaves the ovum in a more or less perfectly 

 developed megalop condition, with the exception of those genera that belong to the 

 Stenopidce which are hatched in the zocea stage. 



1884. 3 c 



