190 THE ACEPHALA. $$ 178, 174. 
§ 173. 
With the Bivalvia, the mantle exhibits (especially near its free borders), 
contractile motions upon the slightest touch. These are due to numerous 
muscular fibres which traverse in every direction its granular parenchyma, 
but are most abundant in the borders. It contains here, moreover, nerves, 
blood and aquiferous vessels, and in some species, even genital organs. 
The borders of the mantle of the Lamellibranchia are often provided with 
very sensitive contractile tentacles; these are rarely wanting around 
the anal opening, — an orifice which serves also for the respiration.® In 
inany, this anal opening is divided by a septum into a round, superior and 
inferior orifice. The borders of these two orifices are often prolonged 
each into a longer or shorter fleshy tube (Sipho). These two tubes, 
which are often blended together, project considerably out beyond the 
mantle and shell, but usually can be wholly withdrawn. p 
With the Brachiopoda, the border of the mantle has, instead of retract- 
ile tentacles, — hyaline, radiating filaments, which are hollow and deeply 
inserted in the substance of the mantle. 
With the Lamellibranchia, and Brachiopoda, the internal surface of 
the mantle is covered with ciliated epithelium, which extends also upon the 
abdomen, foot, oral tentacles, and branchial lamellae. 
This epithelium is of great importance, since it constantly directs cur- 
rents of water into the mantle, and thereby food is brought to the mouth, 
fresh water to the branchiae, the eggs and sperm are carried away from the 
genital openings, and the faeces are rejected outwardly. The existence of 
this epithelium makes it clear how these animals can continue to live when 
buried in wood or stone. 
§ 174. 
The mantle of the Bivalvia is covered by two shells, whose infinite vari- 
ety of form serves for their zoological classification into genera and species. 
These shells are composed for the most part of carbonate of lime so closely 
With many of the compound Ascidiae, the body siphon is double and very protractile. With Cy-- 
sends fleshy ramified prolongations into the man- 
tle. These have been regarded as blood-vessels by 
Savigny (Mém. &c. p. 47, (Diazona and Botryl- 
dus)), and Delle Chiaje (Descriz. &c. III. p. 34, 
Tav. LXXXIIL. fig. 13,15 (Polyclinum viride)) ; 
but Milne Edwards (loc. cit. p. 41, Pl. VIL. fig. 1, 
1b. Je. 6d.) has regarded them with Botryllus 
rotifera, and Didemnum gelatinosum, as hollow 
prolongations,— a view entirely assented to by Kél- 
diker. 
1 With Avicula, Anomia, Pecten, and Spon- 
dylus, there are two or three rows of cylindrical 
tentacles along the border of the mantle; with 
Lima, these tentacles are highly developed, and 
are situated upon the convex edge of the fold of the 
mantle. With Mytilus edulis, they are peculiar, 
being flattened and digitiform. 
2 With the Naiades (Unio and Anodonta), there 
are no tentacles around the anal fissure, while the 
principal mantle-orifice which is separated from 
this last by only a narrow isthmus, has them quite 
numcrously upon its borders ; see Pfeiffer, Nat- 
urg. deutsch. Land-und Siisswasser Mollusken, 
Abth. II. Taf. I. fig. 2, 5,9, p.h. These Naiades 
have also a third fissure, which is dorsal and situ- 
ated quite distant from the anal one; it was first 
pointed out by Bojanus. Iam yet unsettled as to 
‘ss nature. See Pfeiffer, loc. cit. Taf. I. fig. 5, t. 
3 Isocardia, Tridacna, and Chama. 
t With Psammobia, Tellina and Venus the 
clas, and Teredo, the two respiratory tubes are: 
more or less blended together at their base ; and 
they are united so as to appear as a single organ 
with Mactra, Mya, Panopaea, Solen, Pholas, Lu- 
traria, Clavagella, and Aspergillum. 
In these two last genera, the mantle is prolonged 
directly into a siphon without any appreciable line 
of separation. It is almost entirely closed, and 
beside the siphon and the narrow anterior open- 
ing, there is in the middle of its ventral border, a 
very small aperture, whose nature is yet with me 
doubtful ; see Riippell and Leuckart, Neue wirbel- 
lose Thiere des rothen Meeres. p. 41, Taf. XII. 
fig. 4,a; and Owen, On the Anatomy of Clava- 
gella, in the Transact. of the Zool. Soc. London, I. 
p. 270, Pl. XXX. fig. 13, 14, or the Isis, 1836, p. 
440, 1837, Tab. II. fig. 13, 14. 
5 These filaments appear to be composed of a 
horny substance. They are smooth and very 
small with Terebratu/la, and very long and jointed 
with Orbicula and Lingula. 
With Orbicula, each article of the filament is 
surrounded with short bristles; see Owen, Trans. 
Zool. Soc. p. 147, 154, Pl. XXII. XXIII.; or the 
Isis 1835, p. 144, 151, Taf. V. VI.; or in the 
Ann. d. Sc. Nat. IIL 1835, p. 55,.66, Pl. I. II. ; 
and Vogt, Neue Denkschriften der allg. schweizer~ 
ischen Gesellschaft fir du g' Naturwis— 
senschaften, loc. cit. p. 8, Taf. I. 
