292 BULLETIN OF THE 



the various parts of a tunicate animal lies in the fact that it is com- 

 posed of a number of nearly concentric tunics or sacs, which are con- 

 nected with each other in a rather complicated manner ; and the 

 names given to these parts by the various authors are not always used 

 in the same way; the names used by one being applied by another to 

 totally different organs, so that it is difficult to understand what part 

 a name is intended to designate unless the terms used are defined 

 carefully. The precise term used is of minor importance, provided its 

 meaning is clearly stated, and since Huxley, in his paper on Pyro- 

 soma,* has defined, with great clearness and exactness, all the terms 

 which he uses, I shall employ his nomenclature as far as possible. 



The Test. — The outer wall of a Tunicate is the "cellulose test" 

 (Figs. 1 and 3, a). This is a sac with two openings, — the "branchial 

 aperture " e, and the " atrial aperture " g. 



The Outer Tunic. — Within the cavity of the test, and united to the 

 latter at the two openings, is the "outer tunic" (Figs. 1, 2, and 3, b). 

 This also is a sac with two openings, and usually conforms to the 

 shape of the inside of the test, to which it may or may not be united ; 

 whenever the two are separated over a considerable area, there is, of 

 course, a chamber between them, but as this is not one of the true 

 cavities of the body and is of no homological importance, the test may 

 be regarded as enclosing no especial cavity. The outer tunic is usually 

 more or less muscular, and is often spoken of as the "muscular 

 tunic " ; it is the " second tunic," of most writers. Its cavity — that is, 

 the space between its inner surface and the outer surfaces of the 

 tunics within it — is the true "body cavity," and since all the blood- 

 channels of Salpa and the forms allied to it are parts of this "body 

 cavity," more or less shut off by the union of portions of the outer 

 tunic to those within, it is often convenient to speak of it as the 

 " sinus cavity," or " sinus system." Within the outer tunic are the 

 "branchial sac," with its diverticulum, the digestive organs; and the 

 " atrial tunic." 



The Branchial Sac- — During the earlier stages of development the 

 branchial sac is an entirely closed chamber surrounded by a tunic, 

 which is in turn entirely surrounded by the body cavity, by which it 



* On the Anatomy and Development of Tyrosoma. Trans. Linn. Soc. ; 1S60, XXIII. 

 'pp. 19C-250. 



