GENUS ZOOTHAMNIUM. 693 



the type now under consideration. The illustrations given at Figs. 13 and 14 of the 

 accompanying plate, are reproduced from drawings of the species remitted by 

 Mr. Forrest, who obtained it in some abundance both in the river Avon, near 

 Evesham, and from the neighbourhood of Birmingham. Anticipating that the form 

 might be new to science, Mr. Forrest originally proposed to confer upon it the 

 distinctive title of Carchesium septatum. 



Carchesium aselli, Engelmann. PL. XXXVI. FIG. 10. 



Bodies elongate, subcylindrical, slightly constricted beneath the peri- 

 stome, about twice as long as broad ; peristome-border turgid, not dilated ; 

 ciliary disc raised but slightly above its margin ; vestibular cleft continued 

 but a short way backwards ; cuticular surface smooth ; contractile vesicle 

 close to the peristome-border ; endoplast band-like, curved ; zoodendrium 

 stout, branching at short intervals, not annulate or striate, largest colonies 

 including about twelve animalcules, usually two or four only. Length of 

 bodies 1-250". HAB. Fresh water, on Asellus aquaticus. 



Supplementary Species. 



The minute Carchesium pygmaum of Ehrenberg, with animalcules that do not 

 exceed the 1-2400" in their greatest length, is evidently a Flagellate form, whose 

 exact identity cannot be predicated, but is apparently referable to one of the two 

 genera Anthophysa or Codosiga. 



GENUS IX. ZOOTHAMNIUM, Ehrenberg. 



Animalcules structurally identical with those of Vorticella, ovate, 

 pyriform or globular, often dissimilar in shape and of two sizes, stationed 

 at the extremities of a branching, highly contractile pedicle or zooden- 

 drium ; internal muscle of pedicle continuous throughout, not disconnected 

 as in Carchesium. Inhabiting both salt and fresh water. 



As originally instituted by Ehrenberg, this genus was distinguished from 

 Carchesium merely on account of its bearing zooids of two forms and sizes on the 

 same tree-like colony. This character has, however, been found too uncertain for 

 retention, and has given way to the more substantial one, first pointed out by Stein, 

 having reference to the continuity throughout the branching pedicle of the contained 

 contractile muscular fibre. This last-named structural feature confers upon the 

 aggregated communities of this genus, as observed in the living state, a modification 

 of comportment that readily distinguishes them from those of Carchesium ; for 

 while in that genus each animalcule, having a separate contractile pedicle, is able 

 to extend itself and contract independently, the movements of the whole com- 

 munity in the case of Zoothamnirim are affected by that of a single unit, and one 

 contracting all the others follow suit. An additional feature distinctive of the genus 

 Zoothamnium is afforded by the position occupied by the contractile fibre with 

 relation to its hyaline investing sheath. In both Vorticella and Carchesium this con- 

 tractile cord exhibits a spiral, or optically a zigzag, disposition within its sheath, 

 such circumstance accounting for the elegant spiral form assumed by the structure 

 as a whole when contracted. In Zoothamnium, on the other hand, there is no 

 such spiral disposition, the contractile cord being axial and straight throughout : the 

 pedicle in its contracted state, as a necessary sequence, may present a plicate but 

 never a symmetrical spiral flexure. Examined with a high magnifying power, the 

 central cord in this genus, as first observed by Ehrenberg, displays a faintly longi- 

 tudinal striate aspect, and is pronounced by Claparede and Lachmann, and more 



