328 MOLLUSCA. 



thickened and frequently folded and beset with tentacles, papillae, 

 and eyes. 



A head, properly so-called, is absent in this class, the parts of 

 the body in the neighbourhood of the mouth being devoid of 

 sense-organs. These are placed mainly on the mantle edges, which 

 are the parts of the body in closest relation with the external 

 medium. 



The edges of the mantle may be entirely free from each other 

 (those forms in which the gill-filaments are not connected to the 

 mantle, and in which there is no concrescence of the mantle-lobes, 

 e.y., Nucula, the Anomiidae, Arcidae, Trigoniidae, Pectinidae); or 

 they may be united to one another indirectly by the attachment 

 of the branchiae, e.g., Unionidae, Ostreidae; or they may be fused 

 with one another in one, two, or three places. When there is 

 only one fusion it separates off the opening of the cloacal or supra- 

 branchial chamber from the general mantle-opening (Mytilidae, 

 Fig. 256, Carditidae, Astartidae, Crassatellidae, most of the Luci- 

 nidae, etc.). When there are two fusions as there are in Yoldia 

 and Leda, and in most of the Eulamellibranchs, and in Septibranchs 

 the one separates off the cloacal opening as in the forms with 

 one fusion, while the second is near the first, and with it bounds 

 an opening adjacent to the cloacal opening. This second opening 

 is the branchial opening; it leads into the general mantle-chamber 

 (Cardlum, Fig. 255). In such forms there is a third opening the 

 pedal opening in front of the second fusion, through which the foot 

 can be protruded. The size of the foot-opening is in inverse pro- 

 portion to the extent of the second fusion. When the second fusion 

 is much elongated, there may be a fourth opening between the 

 pedal and branchial orifices (Solen, Lutraria, and some Anatinacea). 

 In some cases, at any rate, this fourth opening is in relation to 

 the byssus, for in Lyonsia the byssus filaments project through it. 

 The further forwards the fusion of the two mantle-lobes extends, 

 the more marked becomes a peculiar elongation of the posterior 

 mantle region round the inhalent (branchial) and exhalent (cloacal) 

 openings an elongation of such a nature that two contractile 

 tubes or siphons (Fig. 251) become formed (especially in boring 

 and burrowing bivalves). These may be so large that they cannot 

 be drawn between the posterior edges of the gaping valves of 

 the shell. The two siphons are often fused with one another, 

 but the two canals, with their openings surrounded by tentacles, 

 remain separate. In the most extreme cases the siphons are 



