156 ZOOPHYTES. 



Although the Botryllus, fig. 1, was superficial, as represented else- 

 where, it formed thicker masses. 



Another specimen invested the valves of a shell united by their hinge. 

 Some parts of both surfaces were covered, and the specimen thickened 

 considerably where the shells gaped in front. 



Fine arborescent crystallizations, different from those afforded by sea- 

 salt, occupied the intervals between some slices of the Botryllus laid on 

 glass slips, appearing after the humidity evaporated ; thus indicating the 

 presence of other elements. 



The rapid decomposition of the preceding specimens, for this product 

 is of difficult preservation, obstructed that continued course of investiga- 

 tion which should chiefly interest the naturalist. The water wherein 

 they lay was tinged greenish ; and when the investing substance had dis- 

 appeared, it proved to have covered so many oyster shells now left bare. 



Frequently the higher orifice only is conspicuous, not the lower com- 

 municating with the central cavity. But there are various obscurities in 

 the structure of this compound Ascidia, which I cannot pretend to ex- 

 plain. 



§ 2. In the month of January a small specimen, composed of no more 

 than four whitish specks, occurred on a leaf of one of the FInridca; with 

 which I had supplied a young Aplysia for food. 



About two months later, this specimen had undergone much altera- 

 tion ; and on April 11th, I found it had extended to a quarter of an inch ; 

 that no distinct system being formed, it consisted of eleven pair of parallel, 

 vigorous Ascidise, discharging dark pellets. 



Some days previously, the extension of the under surface over the 

 edge of the leaf, attaching it to the vessel where it lay, shewed how fit 

 a subject it might be for observation, if a suitable position could be obtain- 

 tained. Therefore detaching it very carefully from the spot, I committed 

 it to a pure and transparent watch-glass, in the expectation that consequent 

 adhesion and diffusion would afford a favourable exposure of its structure. 



I was not disappointed. The specimen began to adhere in forty- 



