ANATOMY AND PHYSIOLOGY. 21 



mining the presence of three tunics, or envelopes namely, 

 the test or outer tunic, the mantle or inner tunic, and the 

 lining- membrane or inner tunic of Prof. Huxley.* The lining- 

 membrane and mantle are always, to a greater or less extent, 

 adherent to each other, and have, except where there is an 

 abdomen developed, all the viscera and the lac unary portion 

 of the blood-system placed between them. On the other hand, 

 the mantle and test in A*ei<lia and Molyula are always free, 

 except at the distal extremity of the respiratory tubes, where 

 they are united ; there is also an attachment at the point 

 where the vascular trunks enter the test. But in the genus 

 Styela (Savigny's third tribe of Cynthia) the test is always 

 more or less firmly attached to the mantle throughout, though 

 at the respiratory orifices the adhesion is greatest. In all the 

 species, however, that have been examined, with the exception 

 of one (a small undescribed species [$. humilift, sp. nov.]), 

 these two envelopes maybe separated without much difficulty 

 in specimens preserved in spirit. In the exceptional case 

 alluded to the mantle is exceedingly delicate; and hence prob- 

 ably arises the difficulty of separating it from the test. In 

 this genus, as well as in Axcnlia and Molyula, blood-vessels 

 pass from the body to the test. It is therefore likely that 

 vessels will be found ramifying in the outer tunic in all the 

 simple Ascidians. In Pelonaia the adhesion of the mantle 

 and test is not by any means so remarkable as was originally 

 supposed ; and, indeed, in this form they are as easily divided 

 as they usually are in Styela. Also in Clavelina these two 

 tunics are slightly adherent throughout, while in Salpn they 

 appear to be as free as they are in Ascidia.-^ 



It should be mentioned that, in a living state, unless the 

 mantle be violently contracted, there is no actual vacant space, 

 <>r space filled with fluid, as has been asserted, between it and 

 the test ; even in those species which have these tunics com- 

 paratively free the two siirfaces lie in close contact. When 

 the animal is dead, however, and preserved in spirit, the body 

 enclosed in the mantle does not by any means occupy the 

 entire space within the test, but lies somewhat shrivelled, and 

 frequently quite free (fig. 1, p. 22), just as commonly happens 

 with the animal of the Lamellibranchs within its shell under 

 similar circumstances. 



This tunic was first pointed out by M. Milne-Edwards, in his work on 

 the ' Ascidies Composees/ p. 54. 



t I have examined only one species of Salpa, namely 8. spinosa, and the 

 specimens were preserved in alcohol. 



