Anatomy of the Lamellibranchiata. 579 



formed by the mantle, which also contains certain pectinated 

 organs for respiration, &c, being destitute of any distinction 

 of sex, — may be divided into such as have a calcareous shell 

 or valves* secreted by the mantle, (Conchifera) ; and others 

 which are surrounded by a membranous envelope, (Tunicata).f 

 The conchiferous or bivalve Acephala may be subdivided in- 

 to the Brachiopoda, in which the mantle has an upper and a 

 lower lobe or lamina, secreting a superior and an inferior 

 valve ; being destitute of any elastic cartilage to open the 

 shell ; the mouth situated under the upper lobe of the mantle, 

 and furnished with two muscular, extensile, pectinated arms; 

 the rectum opening laterally, and respiration effected by the 

 internal surface of the mantle ; — and into the Lamellibranchi- 

 ata, Blainv., in which the animal is compressed in a different 

 direction ; having two lateral lobes to the mantle, secreting a 

 right and a left valve ; having commonly an elastic interval - 

 vular cartilage ;X the mouth opening anteriorly at the junc- 

 tion of the lobes of the mantle ; the rectum in the opposite 

 direction ; and commonly furnished with four fixed, mem- 

 branous, pectinated tentacles at the mouth, and four others 

 for the purpose of respiration. To the anatomy of the latter 

 order the author trusts the following pages will not be an un- 

 acceptable addition. 



The Anomia is a genus intermediate between the two or- 

 ders. The structure of it, in several respects, approaches that 

 of the Orbicula.^ In the latter, however, the apex of the 

 upper valve is nearly central, whilst, in the Anomia, it is mar- 

 ginal. || The Anomia has this valve destitute of the wider 

 portion of the former, between the apex and the margin. — 

 Were this part, and the corresponding portion of the mantle 

 supplied, the mouth would then be situated in the same po- 

 sition under the upper valve. Two of the ordinary tentacles 

 of the LamellibranchiatahsLvebecome exceedingly long, and 

 nearly free, also of a similar structure. There is under the 



* The two pieces composing the shell are called valves. 



f In some of the Tunicata there are calcareous pieces, though overlooked 

 by Savigny. They consist of two small, conical tubes, curiously reticulate 

 in their structure, situated one in each orifice of the cartilaginous tunic, and 

 projecting from it externally. The water &c. must pass through them. — 

 The author has found them in several British species. They are noticed by 

 Eysenhardt, « Nova Acta Acad. Nat. Cur.' vol. xii. 



I The genus Myastropha of Gray has no cartilage, but a muscle in its 

 place : some of the Pholades, Teredines, &c. have no cartilage. 



§ See the excellent paper ' On the Anatomy of the Brachyopoda of Cuvi- 

 er,' by Mr. Owen, 'Trans. Zool. Soc' 1833, to which the author is indebted 

 for the anatomy of the Orbicula. 



|| In some species, however, not quite so, according to Turton. 



