§ 194. THE ACEPHALA. 211 
These four branchial lamellae, whose lower border is free while the other 
is attached to the viscera, always extend along the abdomen, and not un- 
frequently come together above. 
Each lamella is formed, essentially, by a widely-projecting cutaneous 
fold, the two leaves of which are connected by numerous transverse septa, to 
which correspond externally as many furrows which pass from the base of 
the branchia to its borders. All these furrows have upon each margin a 
row of long cilia, which, upon the borders of the branchia, connect with an 
ordinary ciliated epithelium.*® The compartments formed by these inter- 
leaved septa are also lined with a very delicate ciliated epithelium, and 
connect with the cavity of the mantle at the base of the branchiae.@”) 
With an entire group of this order, the branchial structure is quite dif- 
ferent from that just described. Externally, these organs appear like ordi- 
nary branchiae, but examined more closely it will be found, that, instead of 
lamellae, they are composed of numerous thickly-set ribands arranged in 
rows."® These ribands are formed of two lamellae blended together at 
their extremity. Their circumscribing space is without doubt solely for the 
lodging of the branchial vessels; for, at their base, there is no orifice anal- 
ogous to those found in the other species of this order. The blood collects 
at the base of the branchiae in the longitudinal canal, from which the lat- 
eral vessels are given off, at right angles. Thence it passes into the branch- 
jae, traversing a trellis-like net-work quite resembling the analogous one of 
the Ascidiae.“® Another series of lateral vessels serves as the branchial 
veins, pouring the blood into other longitudinal canals, whence it passes 
into the two auricles of the heart. 
the different orifices of the mantle may be clearly 
seen by observing these animals, when they, at rest, 
protrude between the valves either their siphon or 
the borders of the mantle and tinge the surround- 
ing water with coloring matter, which makes the 
currents quite distinct. 
15 With Unio, Anodonta, Mactra, Cardium, 
JIsocardia, Lutraria, &c., the four branchial layers 
reunited at their posterior extremity. But with 
Pecten, Avicula, Arca, Pectunculus, and Pin- 
na, they are disconnected and extended backwards 
by two free prolongations. 
16 These ciliary movements tend to carry the wa- 
ter, with the internal branchiae, towards their free 
order ; and with the external, towards their base. 
17 These orifices of the branchial compartments 
are easily seen at the base of these organs; ex- 
cepting, however, those belonging to the two exter- 
nal branchiae,which are concealed beneath a kind of 
‘canal formed by the mantle. These two canals, 
«closed in front, open behind between the end of the 
abdomen and the anus into that portion of the cavity 
of the mantle leading to the anal fissure or tube, and 
which may be regarded as a Cloaca; see Unio, 
Anodonta, Venus, Cardium, Isocardia, Mactra, 
&c. With many, as for instance with Unio, and 
Anodonta, the compartments of the external bran- 
chiae are much more developed than those of the 
internal, and their orifices can be closed in a lip- 
like manner by the vesicular enlargement of the 
septa which limit them on each side. With Pinna, 
the branchiae are quite different. Their leaves are 
united by short filaments instead of by septa; and 
thus, although not divided into compartments, 
there are orifices at their base which lead into the 
interior, and which are situated upon the internal 
surface of the internal, and upon the external sur- 
face of the external branchiae. 
18 This pectinated form of the branchiae has 
been observed by Baer (Meckel’s Arch. 1830, p. 
340), with Mytilus, and by Meckel (Syst. d. ver- 
gleich. Anat. VI. p. 60), with Spondylus, Pecten, 
and Arca ; see, also, the Régne animal de Cuvier, 
nouv. édit. Mollusques. Pl. LXXIV. fig. 2,a. I 
have seen similar b hiae with Pect 
Avicula, and Lithodomus. Philippi (Wieg- 
mann’s Arch. 1835, I. p. 274) has seen them even 
still more developed with Solenomya. 
This peculiar branchial apparatus with Mytilus 
has been described with much detail by Sharpey 
(Cyclop. of Anat. I. p. 621). I have myself, dur- 
ing the autumn of 1847, completely verified the 
statements of this author, upon living specimens of 
the genera Mytilus, Arca, and Pecten, and would 
insist here only on a single remarkable fact. Each 
riband-like branchial filament has, upon both of 
its surfaces, severa] cap-like papillae by means of 
which these filaments are united together in a trel- 
lis-like manner. When the branchiae are forcibly 
distended, the papillae of the filaments are separ- 
ated from each other. But this separation is not 
very extended, for there is a cord composed of del- 
icate fibres, between each two papillae and binding 
them together ; but sometimes, from undue force, 
this cord is broken in its middle, and then each 
broken extremity appears as a bundle of moving 
vibratile cilia which projects from the cavity of the 
capsule (see Sharpey, loc. cit. fig. 305, E.a). The 
function of this apparatus, which ceases to be visi- 
ble when the papillae are united together, is yet 
wholly doubtful. 
1 Unio, Anodonta, Lima, Pinna, Ostrea, &c. 5 
see Treviranus, Beobacht. aus d. Zoot. &c. fig. 62, 
63, 65 (Ostrea and Anodonta) ; and Poli, loc. cit 
Tab. IX. fig. 17 (Unio). 
20 For the branchial vessels, see, especially, Bo- 
lus. 
