182 



HARDWICKE'S SCIENCE-GOSSIP. 



■difference in length is most marked in the anterior 

 pair of legs, employed by the insect in walking ; but 

 it is also observable in the posterior (climbing) pairs 

 of legs. 



I have compared the mount in question with a 

 specimen independently prepared by Mr. W. M. 

 Osmond, a member of our Microscopical Society 

 here, and with two other specimens mounted by me j 

 also with a carefully executed drawing from a German 

 preparation ; in none of them, however, is there any 

 double claw. Mr. Osmond employed caustic potash 

 in cleaning his object ; in all of my slides I adopted 

 Mr. Jackman's process for mounting a tape-worm 

 ■{vide "The Microscope" for January last, p. 5), 

 which, in the few instances in which I have tried it, 



Fig. 77. — Terminal claw of second pair of legs, showing infernal claw 

 attached to muscle, and longer than external claw. 



has answered well for small insects. My objects 

 were perhaps not sufficiently flattened to admit of 

 their being clearly focussed all over, but I am 

 responsible for this feature. Does the insect in 

 question moult, and was this specimen secured at an 

 opportune moment ? 



The photographs which illustrate this note have 

 been expressly taken for it by Mr. Osmond ; it 

 seemed to us that in a matter of this kind photos 

 would be accepted, as being more reliable than hand 

 drawings. 



In two specimens we have observed a long oval 

 body (shown in one of the photos I enclose) : is it an 

 egg? I have a drawing of the abdomen of a choice 

 sample of Cimex lectularius, which contained no less 

 than six unmistakable ova : I examined it in pure 

 carbolic acid as a medium ; and it brought the ova 

 to view with wonderful clearness. 



Calcutta. W. J. Simmons. 



ON THE DISTRIBUTION OF AQUATIC 

 FORMS. 



By T. D. A. COCKERELL. 



IN my previous paper on Geographical Distribution 

 1 dealt almost entirely with terrestrial species, 

 and did not speak of those inhabiting the sea. I now 

 propose to consider some of the more noteworthy 

 facts concerning the distribution of aquatic forms, 

 dealing, as before, mainly with those which inhabit 

 the British Isles. 



The wide distribution of freshwater species is 

 familiar to every naturalist, and was fully recognised 

 and insisted upon by Darwin,* who attributed it 

 mainly to the agency of birds. Some are 

 universal, as for instance Charafragilis, which 

 is "found in every country and clime, in ice- 

 water at the north, and in the hot springs 

 (boiling water) of the Yellow-stone." f But 

 here it must be remembered that Chara is a 

 Cryptogam, and the universal distribution of 

 many terrestrial cryptogamia is well known. 

 Mr. C. R. Orcutt has sent me an interesting 

 list of the flora of Southern and Lower 

 California, which includes a large proportion 

 of freshwater plants common to Britain ; for 

 instance, the three species of Potamogeton 

 quoted are natans, pusillus, and lucens, and 

 iV\ / the three Lemnacece include Leiuna trisulca 

 Wm and L. minor, species which I have also found 

 in Saguache Creek, Colorado. Callitrichc 

 vcrna is likewise a Califomian plant. 



Freshwater sponges are widely distributed, 

 one of the most extraordinary instances being 

 that of Meyenia phtmosa, which occurs of the 

 typical form in Bombay, and reappears as 

 the var. Palmeri along the banks of the 

 Colorado River, in North- Western Mexico. 

 A genus of Trichoptera, Helicopsyche, is 

 world-wide, being found in Europe, America, New 

 Zealand, &c, and the larvae of these insects live in 

 streams, encased in a shell, resembling very closely that 

 of the Molluscan genus Valvata. Probably these larvae 

 are distributed by birds, as it is incredible that the 

 perfect insect should have the power to cross wide 

 tracts of ocean, or even great distances on land ; and 

 it has come under my own observation in Colorado, 

 that wild ducks {Anas bosca) greedily devour these 

 larva-cases, containing the living larvae. 



But it must not be supposed that freshwater species, 

 even the most widely distributed, are not prone to 

 vary. 



Every conchologist is aware of the immense and 

 puzzling variation in such genera as Limncea and 

 Pisidium, not so much in species whose range is 



'Origin of Species," 6th ed. 1882, pp. 343-347- 

 ' Kotanical Gazette," 1888, p. 67. 



