TII MOLLUSC A INTEGUMENT, MANTLE, VISCERAL DOME 49 



D. Seaphopoda. (Cf. Review of Outer Organisation, p. 34.) 



E. Lamellibranehia. 



From each side of the body there typically hangs a large leaf-like 

 mantle fold of the same shape as the shell-valve formed by it. These 

 mantle folds project beyond the body in front, below, and behind, and 

 enclose a mantle cavity which everywhere, except dorsally, opens 

 outward by means of the slit left between the edges of the folds. 

 This large single cleft serves for the admission of nourishment and 

 water into the mantle cavity, and for the expulsion of the excreta, 

 genital products, and respired water; through it also the foot is 

 protruded. Such a primitive mantle is thus completely open, its 

 simple edges (i.e. without folds, papillae, tentacles, or eyes) are quite 

 free, coalescing nowhere. 



The above serves for a description of the mantle of Nucula 

 one of the Protobmncliia and must be considered as the primitive 

 arrangement. 



In most Lamellibranehia, however, special differentiations of the 

 margin of the mantle occur ; these take the form of folds, thickenings, 

 protuberances, papillae, tentacles, glands, eyes, etc., and this is the 

 case both in forms which have an open mantle and in those in which 

 the mantle is partially closed. 



The partial closing of the mantle is brought about by the con- 

 crescence at one or more points of the free edges of the mantle 

 folds. 



A. A completely open mantle, i.e. one single large cleft entirely separating the 

 edges of the mantle, is found : 



(a) Among the Protobranchia in Nucula. 



(b) Among the Filibranchia in the Anomiidce, Arcidce, Trigoniidce, and a few 

 Mytilidce (Pinna). 



(c) In all Pseudolamellibranchia except Meleagrina. 



(d) Among the Eulamcllibranchia, only in a few species of Crassatclla. 



B. The mantle folds of the two sides grow together at one point. In this 

 case the point of concrescence almost always lies high up posteriorly ; and marks 

 off a small aperture from the originally simple cleft. This aperture, occurring on 

 a level with the anus, forms the exhalent or anal aperture of the mantle. Its edge 

 may be more or less prolonged posteriorly to form an anal siphon, which can be 

 protruded beyond the valves of the shell. 



At a point a little below this exhalent aperture, the mantle edges usually become 

 applied to one another, although no concrescence takes place. Above this point, 

 between it and the anal siphon, they separate to form an inhalent or branchial 

 aperture. The edges of this aperture also may be produced posteriorly into a 

 branchial siphon, which, however, in this case, has a cleft extending along the 

 whole of its lower side, which is a continuation of the large cleft of the mantle. A 

 branchial siphon formed in this way, by mere apposition of the mantle edges, is found 

 in the genus Malletia among the Protobranchia. 



VOL. II E 



