ALIMENTAEY CANAL OF TUNICATA. :401 



its rows of slits ; as well as ontgrowtlis of diverse kinds, wliich 

 are sometimes ridge-like, sometimes papilliform, and give rise to 

 numerous complications by the formation of anastomosing processes. 

 The most remarkable are the tongue-like appendages {" languets ") 

 found in Ascidians and in Pyrosoma, which form a long row along 

 the dorsal surface. Opposite to them lies the ''ventral groove/' 

 already mentioned above. 



The branchial chamber of the Ascidians appears, from what has 

 just been stated, to be, in respect of the structure of its walls, a 

 very different organ from that of the Appendiculariae, and must 

 have been formed only after a long series of modifications. 



The same is essentially true for the other Acopa. The Pyrosom^e, 

 in other respects very closely allied to the Ascidians, present an 

 aboral position of the cloacal opening in conjunction with the arrange- 

 ment of the several persons forming a colony around the walls of 

 a hollow cylinder. The persons arranged in the wall of such a 

 cylinder are placed with their incurrent orifices on the outer surface, 

 whilst the cloacte open into the cavity of the cylinder, the orifice of 

 which serves as the common outlet of all the cloacfe. 



In the Cyclomyaria the body, which in its mature form is tub- 

 shaped, has a wide internal cavity. The gill, which traverses this cavity 

 obliquely, and is formed by a membrane perforated by a pair of clefts, 

 divides the internal cavity into an anterior and a posterior division. 

 The anterior is the branchial chamber, into which the incurrent 

 orifice leads ; the posterior space, into which the mass of the viscera, 

 covered by the body- wall, protrudes, is the cloaca, and corresponds 

 to the chamber formed by concrescence around the primitive branchial 

 chamber in the Ascidians. The Salpae have a similar disposition of 

 parts. The gill is, however, in them more completely detached from 

 the wall of the respiratory chamber, and forms a rafter stretching 

 obliquely from the dorsal wall of the respiratory chamber in front, 

 to the ventral wall behind, on each side of which the respiratory 

 chamber is widely open to the hinder space representing the cloaca. 

 The excurrent orifice proceeding from this has a more dorsal position, 

 and is not unfrequently drawn out into a tube-like form (Fig. 212, h). 

 In consequence of the reduction, in this case, of the gills to the 

 rafter-like septum, there is no formation of actual gill-slits, and the 

 water taken into the branchial chamber streams laterally on each 

 side of the median branchial septum into the cloacal chamber. 



The taking of water into, and its ejection from, the body, has in 

 both Cyclomyaria and Thaliadae a close connection with locomotion. 

 This function is here in fact connected with respiration, and the 

 position of the incurrent and excurrent apertures are consequently 

 of importance. 



The water taken in in front is, after it has passed the respiratory 

 chamber, driven on towards the aborally-placed excurrent aperture by 

 the action of the muscular rings of the body-wall, and the stream thus 

 expelled works as a vis a tergo, and moves the body forwards by 

 jerks. 

 ^ 2 D 



