ASCIDIA. 143 



considerable part of the upper portion of the specimens is free. If any 

 occur entirely loose, it should be ascribed to accident. 



The young are nearly transparent. A month or two after a small 

 one of this description came into my possession, it was removed from its 

 position where the adhesion was not strong. Being about half an inch 

 long, it remained free another month, when fleshy prongs generated from 

 the lower part of the side now affixed it to a new substance. Its size in- 

 creased considerably during survivance for eight or ten months, however the 

 slightness of the adhesion allowed it to be detached. — Fig. 5. 



Shelter seems indispenable for the security of this species. The skin 

 grows dark in time, and peels off if distempered. External changes are 

 more conspicuous here than in those originally of darker hue. The trans- 

 parency of the animal adapts it well for observation. Almost the whole 

 line of the intestine is frequently visible, as occupied by the muddy mass 

 from which sustenance is eliminated. 



The Ascidia intestinalis is common in our seas. 



Plate XXXIV. Fig. 4. Ascidia intestinalis. Orifices a. It. 

 5. Young. 



§ 3. Ascidia rustica. — Plate XXXIV. Fig. 6. — Of all the Scotish 

 Ascidia? this is of the most frequent occurrence ; and it dwells in situa- 

 tions the most different, as well as occupying the greatest variety of sub- 

 stances. To each of these it is affixed by the base alone, and rises about 

 an inch and a half, or somewhat more, in height. It is rather of ovoidal 

 form, with two low serrate orifices ; and it is totally invested with a thick 

 coriaceous integument. Larger specimens are reddish, or mottled red 

 and grey, or white. Smaller subjects are distinguished by these latter 

 colours. 



In early stages the animal is comparatively transparent. If then de- 

 tached from some softish substance' whereon it is sometimes found, which 

 may be easily done, and freed of impurities, it proves an interesting object 

 for microscopical observation. 



One of this description, a line in length, small and white to the naked 



