210 $ 194. 
THE ACEPHALA. 
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 fespiratory 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 off from the base of the respiratory cay- 
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 branchia. The internal surface of the halves of this organ is occupied 
with a system of very apparent blood-canals. 
With Terebratula, and Orbicula, 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 
pairs 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 These ciliated organs were first described by 
Meyen, loc. cit. p. 385. 
of the branchial 
partly known this arrang 
vessels. 
6 With the compound Ascidiae, the arrangement 
is such that several individuals are disposed in a 
star-like manner abouta cavity in which their anal 
tubes open. 
7 Cynthia microcosmus, momus, &c. 
8 See the figures of Savigny, and Milne Ed- 
wards, loc. cit. 
9 Phallusia suleata, monachus, intestinalis, 
and Diazona violacea ; see Savigny, loc. cit. Pl. 
1X.-XIL. 
10 See Milne Edwards, Sur les Ascidies compo- 
sées, p. 7. Cuvier, and Savigny, had already 
11 Owen, loc. cit. 
12 See Cuvier, Owen, and Vogt, loc. cit. 
13 The two external branchial lamellae are usu- 
ally alittle smaller than the two internal; and this 
difference is well marked with Cardium. Accord 
ing to Valenciennes (Comp. Rend. XX. p. 1688, 
XXI. p. 511), there is only a single pair of branchiae 
with Lucina jamaicensis, and columbella, Cy- 
therea tigerina, Tellina crassa, and Svlen radi- 
atus. In this last species, they consist only of two 
narrow, longitudinal swellings. 
14The ingress and egress of the water through 
