286 Mr. A. Hancock on a species of Hydra 



prehension, according to Corda, who appears to have examined 

 these organs with great care, but whose description of one of 

 them is erroneous in several respects : the original memoir, how- 

 ever, I have not seen. Neither have I had an opportunity of 

 consulting Ehrenberg^s account of the minute structure of the 

 tentacles of these animals. The smaller vesicles, PL VII. fig. 

 7 a, and fig. 8, seem to agree with Corda's description as given 

 in Johnston's ^British Zoophytes'; they are elliptical, being com- 

 posed of an inner and outer sac, both very delicate, transparent 

 and membranous. These bodies are placed with their long axis 

 perpendicular to the sui'face of the nodule in which they are 

 imbedded, and have a non-contractile hair-like process projecting 

 from the external end. 



The larger vesicles, fig. 7 b, are very complicated, and appear 

 to have been only partially understood by Corda, though he had 

 arrived at a full comprehension of their functions. They are 

 not merely prehensile organs, but are undoubtedly also stinging 

 instruments, as supposed by this naturalist, and are at least twice 

 as large as those for touch : they, fig. 9, are of an ovate form, 

 short and stout, immersed in the substance of the nodule with 

 the narrow end uppermost, and immediately beneath the surface. 

 At the bottom of the vesicle, which though perfectly transparent 

 has rather thick walls, is seen a delicate lining membrane, dy 

 folded down upon itself, having a cup- or saucer-like appearance. 

 This, according to Corda, is " a saucer-like vesicle.'^ Standing 

 up from the centre of this is a transparent membranous stalk, 

 c, irregularly bulged a little at the sides, and surmounted by a 

 sharp arrow-like head, h, with the barbs much depressed. This 

 supporting stalk is described by the naturalist just quoted as 

 " a solid, ovate corpuscle." But to get a full knowledge of this 

 apparatus, it must be examined when exserted and ready for 

 action. The saucer-like vesicle is then seen to have disappeared, 

 and the outer vesicle, fig. 10 «, is lined throughout with a delicate 

 membrane, b ; in fact the lijis or margins of the saucer- like vesicle 

 have unrolled themselves and now form the upper portion of the 

 lining membrane, the saucer itself being the lower portion of it. 

 And moreover the lining membrane is continued through the 

 neck of the outer vesicle, and is seen to be prolonged into the 

 stalk, c, supporting the arrow-head ; the barbs, c', of which, three 

 in number, are now very much elevated, being almost horizontal ; 

 and thus protruded beyond the surface of the tentacle are ready 

 to lay hold of prey in the manner of a grappling-iron. 



The animal would appear, however, to have the power of 

 throwing the whole apparatus from the tentacle. If a specimen 

 be laid on a piece of glass and examined through the microscope, 

 a number of these organs with the barbs fully extended will be 



