REPORT ON THE TUNICATA. 31 



In the Ascidiidae tlie musculature is much more feebly developed. In typical forms 

 (such as Ascidia virginea, 0. F. Muller) the muscular fibres are almost entirely confined to 

 the right side of the mantle — the part lying over the branchial sac, — while the left half, over 

 the intestine, is thin and membranous. The bundles run in all directions, branch and 

 anastomose so as to form an irregular network of fibres, meeting at all angles. In the 

 genus Ciona the arrangement is more regular. Delicate bundles run circularly, and form 

 a thin non-continuous coat, while much stronger bundles run longitudinally, and are 

 united together into twelve to fourteen strong parallel bands. In Chelyosoma the 

 muscular fibres are united into bundles placed between the edges of the horny plates into 

 which the upper part of the test is modified. 



In the Clavelinidse the mantle is thin and the muscular system extremely feeble. 

 The bundles are delicate, and placed far apart, and they run chiefly longitudinally. 



In the Ascidise Compositse also, the mantle is thin, and the musculature delicate. 



In Pyrosoma the mantle is delicate and the muscle bands are confined to the 

 neighbourhood of the branchial and atrial siphons. 



In Doliolum the muscular fibres are arranged in eight or nine distinct bundles, 

 which form complete hoops encircling the body, and by their contraction expel the water 

 in the branchial and atrial cavities through the terminal apertures ; thus propelling the 

 animal through the sea. 



In Salpa the mantle is thin and tough, and the musculature is developed in the 

 form of a number of strong transverse bands, which sometimes branch and unite, and 

 sometimes remain distinct, but do not form complete hoops as in Doliolum. Their 

 arrangement is most definite and characteristic. There are also sphincters round the 

 two terminal apertures. 



In the Appendiculariidse the only muscles known are those of the caudal appendage, 

 where there are two large bundles of striped muscular fibres placed at the sides of the 

 urochord. 



Branchial Sac (Pharyngeal Sac, Branchia). 



The branchial sac is probably the most important and characteristic organ of the class. 

 It is a modification of the pharynx or first part of the alimentary canal, and differs widely 

 in the different groups, thus affording valuable diagnostic characters. It is usually of 

 considerable size, and in most cases is almost as large as the mantle cavity, while in a 

 few (e.g., Ascidia mammilla ,ta), it is longer and has the posterior end folded forwards 

 upon the left side of the body. It communicates with the exterior by the branchial 

 aperture or mouth, which is placed anteriorly, and is either circular (Clavdina) or sur- 

 rounded by a definite number of more or less distinct lobes, varying from three (Chdeolus) 

 up to twelve or fourteen (Abyssascidia). The test is prolonged inwards at the edge of the 



