390 Mr. Hogg's Observations on the Spongilla fluviatilis, 



first called to them by seeing some little extraneous substances floating in the 

 water and being carried by the stream setting in directly to a particular part 

 of the Sponge. With a good magnifier I then clearly perceived not only this 

 current in action, but likewise another flowing from the canals through their 

 large orifices or oscules, and bearing along with it stnall ejectamenta or 

 brownish bits of the Sponge or of soil, which I frequently observed turning 

 over and over, and sometimes kept by the currents in a revolving motion within 

 the oscules, and thereby undoubtedly showing that certain flowings or streams 

 were taking place in the water. I have further established the presence of 

 them by putting a fresh specimen of the Spongilla Jluviatilis in a dish of 

 water and sprinkling a little calcined magnesia on the surface ; this powder 

 soon exhibited motions, and betrayed by its being carried round and round 

 upon the water in different directions, that strong and distinct currents were 

 actively going on. After many minute examinations in endeavouring to solve 

 the cause of these phsenomena, I have hitherto invariably found a worm or an 

 insect, or some crustaceous or molluscous animal, concealed within the Sponge, 

 and by which the currents were actually produced. 



A parasitical insect*, indeed, which I have seldom failed to observe lurk- 



* Mr. Westwood, F.L.S., having more fully described this curious insect in a letter dated Novem- 

 ber 30th, 1838, with which he kindly favoured me, I will add the following interesting account from 

 it : " The insects you have sent me are as anomalous as the production they inhabit. From their 

 small size, soft texture, six long legs, and green colour, they have much the appearance of small 

 wingless Aphides ; but this is only in appearance. On examining them under a lens, they are seen 

 to be 13-jointed, of an elongated oval form, clothed at the sides, and especially at the extremity of 

 the body, with very long setae, which are also found sparingly upon the legs. The latter organs are 

 not those of a perfectly organized imago, for they resemble those of the immature Coccidcs, or rather 

 the legs of the larvse of the HemeroUi, &c. The antennae are nearly half the length of the body, 

 very slender, arising from an incrassated basal joint, and apparently very indistinctly articulated. The 

 mouth consists only of four exceedingly fine setae, as long as the anlennBe, between which they are 

 constantly porrected, arising in pairs at a short distance apart. The underside of the body, or at least 

 some of the terminal segments are furnished with elongated, flattened, membranous appendages ; 

 which, from analogy with some of the aquatic Neuropterous and Trichopterous larvae, are evidentlv 

 false gills, and employed in respiration. From the characters, and with the materials I now possess, 

 it is impossible to decide upon the real nature of these little animals : neither is it possible to decide 

 whether they are in the larva or imago state. There are some imagines amongst the Aphidm, Coccidx, 

 and some other groups, which never gain wings, but always retain their larvalike form ; and in the 

 Coccida we have a mouth composed of four elongated setae, as I have discovered ; but then these 



