146 ZOOPHYTES. 



expose the interanea obscurely. On the whole, I concluded that it might 

 be a young Ascidia rustica. The skin continued spreading more than 

 usual, on the glass vessel containing it. After the animal had survived 

 four months in vigour, nothing remained on subsequent inspection but an 

 empty sac. 



When the Ascidia scabra decays, an empty transparent bag, the ex- 

 terior coating, is left, being voided of its contents. 



It is thus that the ascidian race is constituted a distinct class, de- 

 nominated tunicata, because the integument is, in fact, a bag, besides which 

 another envelope includes the vital organs. 



Platk XXXVI. Fig. 2. Ascidia scabra. 



3. Ascidia scabra, young I 



4. The same, enlarged. 



§ 5. Ascidia mentula. — Plate XXXVI. Figs. 5, 6. — Compared 

 with the preceding, and all others that I have seen, this species attains 

 gigantic size. It is the largest of its tribe. Specimens obtained in the 

 Orkney islands, are no less than eight inches in height, and three in their 

 widest diameter. There is more inequality in the relative position and 

 proportion of the parts here, than in most of the rest in the Scotish seas ; 

 the higher orifice terminating that portion of the body which is five inches 

 above the lower orifice. The whole exterior is very hard and coriaceous, 

 of irregular conformation : much resembling an old root, or part of the 

 stunted, decaying branch of a tree. Those of large dimensions are covered 

 completely with fuci and other parasite substances, finding an appropriate 

 foundation in the rugosities of the surface. 



But specimens of this description are seldom found. They seem to 

 belong only to what are esteemed the more remote parts of Scotland : 

 Nor are they common there. I have seen but few. 



I was indebted for a fine specimen to the favour of Dr Duguid, a 

 learned physician in Kirkwall, Orkney, always ready to promote the re- 

 searches of the naturalist. This animal, perfectly clean and vigorous, ex- 

 tended three inches in length, by an inch thick where of greatest diameter. 

 The distance between the orifices, though great, did not appear quite so 

 disproportionate as in those of unwieldy growth. — Plate XXXVI. Fig. 5. 



