210 THE ACEPHALA. <§> 194. 



of the branchiae with those of the body, and at its opposite one with the 

 heart. Externally, it is covered with large cilia.*'' 



2. With the Ascidiae, the walls of the body are, for the most part, lined 

 with a membranous branchial apparatus. In the place of respiratory or- 

 ifices, there are, what are usually called an oral and an anal tube. By the first 

 of these, the water containing food passes directly into that part of the cav- 

 ity of the body which contains the branchial apparatus, and which is there- 

 fore called the respiratory cavity. By the second, this cavity is emptied 

 of the refuse water containing faeces.*''' 



The branchial membrane, which, in some of the simple Ascidiae,*^ forms 

 numerous longitudinal folds extending entirely over the respiratory cavity, 

 presents a trellis-like aspect with rectangular meshes.'*' These meshes which 

 form prominent lines, have often small fleshy papillae, <-'' and are always pro- 

 vided on each side with a row of very long cilia which produce regular cur- 

 rents of water. 



Two longitudinal sinuses pass ofi" from the base of the respiratory cav- 

 ity and ascend along its greater and lesser curvature even to the oral tube, 

 where they intercommunicate by a circular canal. These sinuses send 

 numerous transverse vessels into the branchial membrane, where they anas- 

 tomose vertically and thus form a net-work corresponding to the trellis 

 just mentioned. 



From the continual changes in the direction of the blood-currents it is 

 impossible to determine which is the arterial and which the venous of these 

 sinuses. <^"' 



3. With the Brachiopoda, the internal layer of the mantle serves as 

 a bi-anchia. The internal surface of the halves of this organ is occupied 

 with a system of very apparent blood-canals. 



With Terebratula, and Orbicvla, there are four large canals upon the 

 surface corresponding to the imperforate valve, and two upon the other sur- 

 face. These arise from two hearts, and are subdivided into numerous mi- 

 nute branches. Parallel to these last, are others, smaller, and which appear 

 to communicate with them on the borders of the mantle ; perhaps they are 

 the branchial arteries, while the larger canals are veins. '^'* 



With Lingula, the branchial vessels are contained in collar-like projec- 

 tions, giving the inner surface of the mantle a very peculiar aspect.''-' 



4. In the cavity of the mantle with the Lamellibranchia, there are two 

 paii's of branchiae, which, as four lamellae, embrace each side of the abdo- 

 men, and the foot.'^^' The water which bathes them comes in partly 

 through an opening in the mantle, and partly by a particular respiratory 

 orifice upon the border of the abdomen, or by the respiratory tube of the 

 siphon. It passes out through the anal orifice, or by another tube of the 

 siphon.*''*' 



5 Thffee ciliated organs were first described by partly known this arrangement of the branchial 



Meyen, loc. cit. p. 385. vessels. 



B With the cunipound Ascidiae, the arrangement 11 Owen, loc. cit. 



is such that several individuals are disposed in a 1^ See Cuvier, Owen, and Vogt, loc. cit. 



star-like manner about a cavity in which their anal 13 The two external branchial lamellae are usu- 



tubes open. ally a little smaller than the two internal ; and this 



7 Cynthia microcosmus, momus, &c. difference is well marked with Cardium. Accurd- 



8 See the figures of Savigny, and Milne Ed- ing to yalenciennes (Comp. Rend. XX. p. 1688, 

 wards, loc. cit. X.XI. p. 511), there is only a single pair of branchiae 



ö Pkallusia sulcata, monachus, intestinalis, with Lucina jamaicensi.t, and Knlumbella, Cy- 



and Diazona violacea ; see Savigny, loc. cit. PI. therea tigerina, Tellina crassa, and Sulen radi- 



IX.-XII. at us. In this last species, they consist only of two 



10 ^fi Milne Edwards, ^Mx les Ascidies compo- narrow, longitudinal swellings. 



•668, p. 7. Cuvier, and Savigny, had already 1* The ingress and egress of the water through 



