G32 MR. E. R. LANKESTER ON SOME LOWER ANNELIDS. 



D 



on some of the larger river species, some specimens of Chcetogaster limncei are to be 

 found. They appear as minute whitish creatures, rarely more than -j^ inch in length, 



■h some chains of zooids, produced by gemmation, may have a length of as much as 

 a sixth of an inch or even more. They crawl rapidly over the surface of the snail's 

 body, and, when one tries to catch them, retire rapidly towards the shell, beneath the 

 velum or tentacles. Their action in moving is like that of a Geometric caterpillar, their 



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bundles of hooked bristles being used like the "feet" of the larval Arthropod. They do 

 not appear to incommode the Limnce'i by their presence, as I have found specimens of 

 the mollusks infested by hundreds, and quite as healthy as those on which none or only 

 one or two of the parasites could be found. Indeed they are probably useful to their 

 bearers, since their alimentary canal is often distended with the Cercarice of the Limnceus, 

 as well as with small Entomostraca, Eotifera, Protozoa, and Protophyta. In the month 

 of December I could find no specimens of Chcetogaster on any Limnceus, nor in January ; 

 in February I found, on a L. stagnalis at Oxford, a well-grown specimen, with a zooid 

 and portions of two growing zooids attached ; it was enveloped in a thick mucous secre- 

 tion of the snail, and was apparently torpid ; in March I have found none ; in April 

 none ; in May all the Limncei appeared suddenly to become infested with them, and con- 

 tinued so until the end of November, when they again completely disappeared *. The 

 same Limncei were not kept and examined from time to time, as it is almost impossible 

 to preserve large individuals of these mollusks in a limited vessel of water; they are so 

 voracious that all vegetation is destroyed by them, and the water becomes putrescent 

 from the abundant fyeces ; the delicate worms rapidly die under these circumstances, not 

 breathing, like their bearers, atmospheric oxygen. The Limncei from one pond at 

 Oxford and one at Hampstead, however, were repeatedly examined, and gave the results 

 stated above. Every specimen of Chcetogaster limncei which I have seen was in a state 

 of gemmation, some with only one not fully grown zooid attached, others with four or 

 eight or portions of more individuals, forming a long chain. Not a single specimen of 

 Chcetogaster in an undoubtedly sexual condition has come under my observation f, nor, 



I believe, under any one's, unless the dubious description and figures of D' Udekem are 

 to be accepted. 



General Ch„m C ters.~A single individual of Chcetoguster limnwi, taken by itself, 

 Without its attendant train of zooids, is less snake-like in outline and less distinctly 

 segmented than most worms, having something of the short stumpy figure of a grub or 

 tardigrade. Its mouth is almost terminal-the dorsal margin being capable, however, 

 of a certain amount of protrusion, like a lip or « prostoinium." There is a pair of bristle- 

 bundles on the ventral surface near the mouth, each having twelve long bristles, with 

 reflected bifid ap lC es ; at some distance from these another series of bristle-bundles 



* 



entered by the renal duct. 



February I have found the CTixtog aster es in *reat numbers 



i Or 



with the peculiar 



and formed very large chains of zooids. Professor 



Hnxh y drew mjr attention to their oecurronee within the Water-snails 



JiKSk'iSta^. IT T' I ^ * '° C0TOCt thc » b ™ "*»«"• ' "™ — fonnd the sexual 

 **«, wh.eh defers m .mportant eharaeters from the asesual. I reserve a demotion of this form. 



