Dr. L. Plate on the Qenus Acinetoides. 203 



what to modify its form, and especially to render the con- 

 vexity of the dorsal surface sometimes quite hemispherical, 

 sometimes much flatter. Further on, in describing the mode 

 of life of our Infusoria, I shall come back to this and indicate 

 the use which the species of Acinetoides make of this faculty, 



Acinetoides Oreeffii is bounded externally throughout by a 

 thin cuticle. The interior of the cell-body is destitute of any 

 special peculiarities. We find in it an elongated nucleus, 

 often curved into the form of a sausage (fig. 1, N), which 

 extends through almost the whole cell, possesses a finely 

 granular structure, and is enclosed by a special membrane ; 

 and, further, numerous granules of fatty lustre, and a contrac- 

 tile vacuole situated close to the ventral surface (figs. I, 2, 

 cv.). 



The second species of the genus, which, from its dwelling- 

 place, may bear the name of Acinetoides zootliamnij resembles 

 that above described in nearly all points. It is, however, 

 considerably smaller, namely about half the size of ^. Gree;ffii, 

 and it possesses a nucleus of different form, spherical and of 

 comparatively very small size. 



The specific difference of the two Infusoria just noticed is 

 also distinctly recognizable in their mode of life. Common 

 to both is that thay reside upon colonies of the Vorttcelline 

 genus Zootliamnium ^ and feed by sucking out the individuals 

 of their colonies. They are therefore parasites and become 

 exceedingly injurious to the elegant structures in question. 

 I have often met with bushes of them which, on some 

 branches, had lost all the individuals by the Acinetoides 

 which swarmed around them. It is remarkable that each 

 species of Acinetoides attaches itself to a definite region of 

 the body of a Zoothamnium. The individuals of Acinetoides 

 Greeffii always select the base of the animal on which they 

 prey at the point where the muscle of the contractile stem 

 radiates in a tuft into the bell (fig. 3), and they fix themselves 

 here by bending the body transversely in the middle, and 

 thus attach themselves closely to their victim. Evidently they 

 are enabled to do this only by the great contractility of the 

 ventral surface, seeing that the attachment is effected by the 

 production of a vacuum within the basin-like ventral surface. 

 While the plasma of the Vorticelline flows over into the 

 Acinetoides, the cilia of the latter are in general quite quiet ; 

 sometimes, however, for a few moments they move again, 

 wholly or partially, a proof that in the attachment only the 

 peripheral border of the ventral surface of our Suctorian is 

 adherent to the prey. In this position the Acinetoides often 

 remains for a considerable time — an hour or more — but fre- 



14* 



