ALCYONIDIUM. 35 



The progress is rather by ascent where investing zoophytes ; and the 

 stratum scarcely exceeds the thickness of a line, where prolonged over the 

 extremity. But in highest perfection, a very vigorous specimen may ex- 

 hibit some short obtuse branches from each side of the portion apparently 

 extended beyond the substance invested. Then the surface seems cellular, 

 the cells tending to a circular figure, and the interior porous. 



The cell is inhabited by a small pale grey ascidian hydra, with about 

 15 tentacula, as enumerated in several individuals. The body is extremely 

 flexible, protruding far, and curving over the dark brown stratum with 

 much vivacious action when displayed. In retreat, it leaves the site of its 

 cell denoted by a pa]filla. 



This animal is probably of delicate nature. Though of frequent oc- 

 currence, it is not often alive. Multitudes drop from their cells, as the 

 purity of the water becomes impaired. 



On one occasion, several weeks after those of an entire colony had 

 perished ; others, which I was induced to consider their successors, issued 

 from the polyparium. 



Among the zoophytes, there are several whose hydras drop readily 

 from the cells. Perhaps this is from greater delicacy, rendering them sus- 

 ceptible of injury by the impure element ; and, farther, the ligaments 

 binding the ascidian species to the interior of their cells, may be also more 

 easily ruptured. 



Another Alcyonidium, of a greyish colour, likewise invests corallines, 

 in a thin stratum. Possibly the substance of the coralline may affect the 

 hue of the zoophyte : or the colour may be partly dependent on the age or 

 the density of the stratum. The hydras of both appear to be in absolute 

 correspondence ; of the two, the dark brown, sometimes almost black, is 

 the more common. 



Whether there be distinctions constituted, by papillae covering the 

 surface of this zoophyte, or by its consisting wholly of depressions ; and 

 whether the latter be accidental, merits investigation. Likewise, some 

 attention should be devoted to a substance much resembling, if not iden- 

 tified with the dark Alcyonidium parasiticum investing shells. 



