December 1, 1899.] 



KNOWLEDGE. 



283 



the firat of the five paka of trimk-Umba are feeble walking 

 lega, whereas in the lobater they carry the well-known 



Second gnathopod of Amphipod, Pnrdalisca ahyssi Boeck, witl\ 

 the bi-anchia and maisupial plate. 



powerful clawa. Also you will find that the second and 

 third masillipeds are not here consolidated at their 

 bases with the other jaws, and that they have 

 exchanged their Latin name tor a Greek one, 

 gnathopods, which means exactly the same 

 thing, only it sounds better, more oracular and 

 Demosthenic. 



Not only in name and function, however, do 

 these gnathopods of the Amphipoda differ from the 

 homologous masillipeds of the lobsters, crabs, and 

 shrimps. In all the latter groups, speaking 

 generally, it will be found that the animal's coat, 

 or carapace, covers the segments belonging to all 

 the mouth-organs, and to all the trunk-limbs. On 

 the contrary, in the amphipod it is a mere jacket. 

 It stops short at the segment bearing the first 

 masillipeds, leaving exposed the segments that 

 carry the two pairs of gnathopods and the five pairs 

 of walking-legs. This is a very essential difference, 

 and implies that the Amphipoda are at a much more 

 primitive stage of development than those crustaceans 

 which have gathered their segments more closely 

 under shelter, and devoted more of their limbs to 

 the service of the mouth. 



High up on the second gnathopods, ajid some or 

 all of the following limbs of an amphipod, there 

 win be found attached semi-pellucid vesicles, a kind 

 of small bags, sometimes flattened, sometimes 

 dilated, sometimes crumpled or twisted, sometimes with 

 other modifications. These are the breathing organs. 



They hang freely in the water. They are sometimes 

 reduced to a couple of rod-like pairs ; but whether 

 more or fewer, they are no doubt nicely adapted to 

 the owner's requirements. When a living amphipod 

 in water is viewed under the microscope, at more 

 or less transparent parts of its skin a number of little 

 globules can be seen, chasing one another merrily about 

 its body. It is, or should be, the ambition of each of these 

 little colourless globules to traverse sooner or later one or 

 other of the branchial vesicles. In such a passage, whether 

 in circuit or struggling through the labyrinth of transverse 

 canals, it will gather from the outside water a new supply 

 of oxygen. Then it will pass into a chamber surrounding 

 the heart, called the pericardium. There it will craftily 

 bide its time. Presently the little side-doors of the heart 

 are opened, and it will pass through with its companions 

 into the august sanctum, thence in turn to be sent forth, 

 through one of the aortic portals, on a fresh career of 

 usefulness. Harvey discovered the circulation of the 

 blood ; he didn't invent it. 



With the branchial sacs of the Amphipoda it looks as if 

 Nature had been trying a series of experiments, first using 

 a small, simple form, then enlarging the surface, puffing 

 it out, adding a fold or two, and finally pleating the whole 

 into a row of leaflets. The object aimed at is to expose 

 as large a permeable surface as possible for procuring 

 oxygen from the external water. In the higher crustaceans, 

 the experimentation conducted on the bodies of the lower 

 bears its fruit. It becomes no longer a question of simple 

 branchial sacs. The forms are described as arborescent, 

 leafy, or hairy — dendrobranchiate, phyllobranchiate, 

 trichobrancbiate. Each branchia is divided up into an 

 immense number of compressed and closely adpressed 

 leaflets, or still more finely sub-divided so that the 

 filaments are comparable to hairs, or there may be a 

 mixture of the two, or a kind of cross between them, with 

 hair-like fringes to a leaf-like base. These more elaborate 

 organs are worth a little extra care, and in the Brachyura 

 and ^facrura they are all neatly arranged out of sight 

 under the carapace. For them, however, " Out of sight, 



Branchial Plume, 



seen in section, 

 arniafus Bate 



of Deep-sea Prawn, Plesiopenoius 

 . From Bate. 



out of mind," would be a very inappUcable motto. Careful 

 order is taken, so that by appropriate orifices water may 



