44< Dr. T. Williams on the Mechanism of Aquatic 



ryngeal from the cloacal chamber. In ultimate structure the 

 branchiae of this genus differ from those of the Ascidians : in 

 place of presenting on each side simple striae furnished with 

 vibratile cilia, as in the Salpians, they bear right and left a series 

 of filiform appendages directed horizontally towards the ventral 

 side of the respiratory cavity, where they are fixed on each side 

 of the middle sulcus, and during their passage across are united 

 together by a number of other slender vertical filaments. From 

 this disposition of parts there results a kind of trellis-work, 

 which fills up all the pharyngeal portion of the branchial cham- 

 ber, permitting no communication between the latter and the 

 cloaca except through the meshes of its network, which are 

 bordered all around with vibratile cilia*. 



The branchial sac of the Botryllidre is like that of the Clavel- 

 linidae : it is similarly organized. The branchial spiracles are 

 variable in number. It is in general only slightly folded. The 

 respiratory sac in Botryllus lies horizontally, and has only nine 

 rows of stigmata, grouped into threes by the longitudinal folds. 

 The angles of the branchial network are marked with papilla in 

 Distoma and Diazona. 



The brancliice in Pyrosoma line the internal tunic of the mantle. 

 They are orally disposed. They consist of numerous vessels 

 or channels anastomosing with each other at right angles. 

 " Nothing is more curious," says Milne-Edwards, " than the 

 respiratory apparatus of these animals, when the vibratile cilia 

 with which each of the stigmata is furnished are simultaneously 

 effecting their vorticiform movements with rapidity and perfect 

 harmony t-" 



In Salpce the gill is constructed of a flattened tube, stretched 

 on a vertical plane obliquely across the central or branchial 

 cavity of the body. It is composed of a double membrane formed 

 by a fold of the internal tunic or mantle. It partitions the bran- 

 chial chamber into two portions — the pharyngeal and cloacal. 



The circulatory systems of the Ascidians resemble that of the 

 Bryozoa. If the heart were removed, it would be a chylaqueous 

 system. It is transitional between the Polypes and the Mollusks. 

 Van Beneden compares the Ascidian to a digestive canal sus- 

 pended in the midst of an external envelope surrounded by a 

 fluid moving in the open spacious perintestinal space. It is 

 only in the branchial network and tentacles that it can be said to 

 be contained in vessels. 



Mr. Gosse gives an exact description of the living circulation 

 in Perophora Listeri (fig. 3). Speaking of the blood-globules, he 



* Sec article Tunicata. — Cyclop, of Anat. and Phys. 



t Annales des Sciences Natuielles, 2nd ser. torn. xii. p. 375 (1835). 



