140 BRITISH TUNICATA. 



This curious genus, wliicli very much resembles a 

 Sipunculus in external form, and might readily be 

 taken at first sight for a vermiform echinoderm, is 

 nevertheless closely related to Stijela in most of its 

 characters. It differs from that genus, however, in its 

 elongated form, in the absence of branchial folds, and 

 more especially in the position of the stomach and 

 intestine, which lie, for the most part, below the bran- 

 chial sac, showing in this respect an approach to 

 Glouvelina and Ciona, and to those compound Ascidians 

 in which the thorax and abdomen are distinct. 



Pdonaia appears to be a boreal form, having yet been 

 found only in the northern portion of Great Britain,* 

 in Norway, and in North America. The species 

 inhabit deepish water on a muddy or sandy bottom. 



A detailed account of the internal structure was 

 given by Forbes and Groodsir in their paper on this 

 interesting genus in the ' Edinburgh New Philosophical 

 Journal ' ; but as the anatomy of the animal was much 

 misunderstood by these two eminent naturalists, we 

 give the following description [of the anatomy of the 

 typical species, P. corrugata] entirely from our own 

 observations. 



The test (PI. XL VI, fig. 15) is not by any means 

 thick ; it is tough, opaque, and firm, with the external 

 surface wrinkled transversely and covered with 

 branched fibrils, most numerous at the inferior, en- 

 larged and rounded, extremity. It is throughout 

 strongly adherent to the mantle, from which it is with 

 difficulty removed. 



The mantle is considerably thinner than the test, 

 and is provided with muscular fibres, of which the 

 longitudinal ones are most conspicuous. It is almost 

 colourless, being only slightly tinged with yellow. 



The branchial sac (PL XL VI, fig. 16; PL XLVII, 

 figs. 2 and 3 ; and fig. 81 in text) is very much elongated, 

 though it does not reach to the bottom of the pallial 

 chamber, the greater portion of the digestive apparatus 



* [Both species, it will be seen later, are recorded from Cornwall.] 



