190 THE ACEPHALA. §§ 173, 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 abuadant 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 

 many, 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 (Slpho). These two tubes, 

 which are often blended together, project considerabl}' out beyond the 

 mantle and shell, but usually can be wholly withdrawn."' 



With the Brachlopoda, 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- das, and Teredo, the two respiratory tubes' are 



tie. These have been regarded as blood-vessels by more or less blended together at their base ; and 



Saviifny (M6m. &c. p. 47, (Diazona and Botryl- they are united so as to appear as a single organ 



lus)), 'And De//e Chiaje (Descriz. &c. III. p. 34, with 3Iactra, Mya, Panopaea, So/en, Pholas, Lu- 



Tav. LXXXIII. fig. 10, 15 (Polyclinum viride)) ; traria, Clavagetla, and Aspergillum. 

 but Milne Edwards (loc. cit. p. 41, PI. VII. fig. 1, In these two last genera, the mantle is prolonged 



lb. Ic. 5il. ) has regarded them with Botrylius directly into a sijihon without anj- appreciable line 



rotifera^axii Didemnum gelatinosum,a.i\io\\ovi of separation. It is almost entirely closed, and 



prolongations, — a view entirely assented to by Kiil- beside tlie siphon and the narrow anterior open- 



liker. ing, there is in the middle of its ventral border, a 



1 With Avicula, Anomia, Pecten, and Span- very small aperture, whose nature is yet with me 

 dylus, there are two or three rows of cyUndrical doubtful ; see Ruppe/l and Leuckart, Neue wirbel- 

 teutacles along the border of the mantle ; with lose Thiere des rotheu Meeres. p. 41, Taf. .XII. 

 Lima, these tentacles are highly develojied, and fig. 4, a ; and Owen, On the Anatomy of Clava- 

 are situated ujion the convex edge of the fold of the gella, in the Transact, of the Zool. Soc. London, I. 

 mantle. With Mytilus e.dulis, they are peculiar, p. 270, PI. XXX. fig. lo, 14, or the Isis, 1836, p. 

 being flattened and digitiform. 440, 1837, Tab. II. fig. 13, 14. 



2 With the Naiades ((77MO and ^nofZoTjZa), there 5 These filaments appear to be composed of a 

 are no tentacles around the anal fissure, while the horny substance. They are smooth and very 

 principal mantle-orifice which is separated from small with TereftraZw/a, and very long and jointed 

 this last by only a narrow isthmus, has them quite with Orbictila and Lingula. 



numerously upon its borders ; see Pfeijfer, Nat- With Orbicnla, each article of the filament is 



urg. deutsch. Land-und Siisswasser MoUusken, surrounded with short bristles ; see Owen, Trans. 



Abth. II. Taf. I. fig. 2, 5, 9, p. b These Naiades Zool. Soc. p. 147, 154, PI. XXII. XXIII. ; or the 



have also a third fissure, which is dorsal and situ- Isis 1835, p. 144, 151, Taf. V. VI. ; or in the 



ated quite distant from the anal one ; it was first Ann. d. Sc. Nat. Ill 1835, p. 55, 66, PI. I. II. ; 



jjoint.^d out by Bojanits. I am yv-t unsettled as to and Vogt, Neue Denkschriften der allg. schweizer- 



its nature. S-e Pfeiffer, loc. cit. T.if. 1. fig. 5, t. ischen Gesellschaft fiir du gesiuumten Natui-wis- 



8 Isocardia, Trid'acna, and Vkama. aenschaften, loc. cit. p. 3, Taf. I. 



4 With Psammobia, TcUina and fenus, the 



