ALCYONELLA. 103 



other features between the Alcyonella and the Fiustra hispida, likewise 

 bearing down among the mud of the sea-shore the leaf of the Fucus serratus 

 invested by it. 



Incredible multitudes of hydra;, resembling the general aspect of those 

 of the Cristatella, whiten the surface of the gelatinous stratum of the Al- 

 cyonella. Their numbers are such, as in some zoophytes already described, 

 to preclude the free exercise of their external organs individually. They 

 offer a confused assemblage to the observer. 



The hydra is very minute, perfectly white, its lunate head provided 

 with from 42 to 44 tentacula, ciliated, as may be discovered with a power- 

 ful magnifier. The body consists of a sac or integument, surrounding the 

 intestinal organs, in the lower part of which a pulsatory or peristaltic mo- 

 tion is seen, and internal particles, evidently tossed up and down, probably 

 indicating fluid circulation. — Plate XXIX. figs. 3, 4. 



After protrusion from the stratum, the hydra sinks amidst it in pre- 

 cipitate retreat, leaving a conical projection above the surface, which ex- 

 hibits some motion. When rising to display itself, and completely un- 

 folded, and in vigorous activity, the cilia of one row bordering the tenta- 

 cula strike upwards, and those of the opposite row on the other side 

 downwards, so that the influence of the whole tends to the same purpose. 

 But high microscopic powers are necessary to shew it. — Figs. 5, 6. How- 

 ever feebly this is done by each, the agitation of such a number as com- 

 poses the whole, produces the attraction and repulsion of neighbouring 

 particles, many of them falling among the tentacula themselves, from 

 whence they must be extricated and carried towards the mouth. A suffi- 

 cient quantity being at length collected in the cavity, the oesophagus 

 dilates, and the mass is swallowed by evident muscular exertion. Its trans- 

 mission to the stomach is distinctly visible. It is only on the accumula- 

 tion of a quantity of matter in the cavity of the mouth, which opens hori- 

 zontally amidst the tentacula, that deglutition follows. The animal's selec- 

 tion is demonstrated by rejection of particular substances. Should a 

 microscopic eel be involved among the tentacula, the hydra endeavours to 

 free itself of the intruder, instead of attempting to swallow it. 



It is difficult to obtain a distinct view of an individual hydra amidst 



