CONCHIFERA. 



707 



between them. It is a circumstance worthy of 

 remark that the siphons are observed to be- 

 come elongated and thickened in proportion 

 as the lobes of the mantle are more extensively 

 united. This circumstance, however, is only 

 true in a general way, for it would be easy to 

 quote many striking exceptions to it. 



2. Siphons. We have already had occasion 

 to see the siphons commence in certain genera 

 by simple perforations ; they increase in length 

 in the succession of genera ; and in a certain 

 number they always continue unconnected 

 through their entire extent fg, h, Jig. 346 ; b, 

 c, fig, 355). In other genera, however, the 

 siphons are seen at first united towards their 

 base, then conjoined nearly to the middle, co- 

 hering almost to their ends, and finally blended 

 through their whole length, so as to form a 

 single elongated subcylindrical fleshy mass, 

 pierced through its entire length by the canals 

 of the two siphons, one of smaller size, situ- 

 ated superiorly for the anus, the other larger, 

 situated under the former, and destined to 

 transmit the water to the branchiae. Whether 

 connected or not, the superior siphon is always 

 characterized as the anal, the inferior as the 

 branchial siphon. 



The structure of the siphons is entirely mus- 

 cular, so that their free extremities are capable 

 of contracting and of being elongated to a 

 very considerable degree. They are beset 

 around their external orifices with a great 

 number of papillae, (w, o,Jlg. 347), occasionally 

 truncated at their extremities and of exquisite 

 sensibility. The water has to pass over these 

 papillae before it can enter the mantle, and un- 

 doubtedly they apprise the animal of the pre- 

 sence of every foreign body that might injure 

 it. In a few genera the siphons contract by 

 means of their component muscular fibres; 

 but in the greater number they have a parti- 

 cular retractor muscle running on each side of 

 the animal, and in relation, in point of mag- 

 nitude, &c. with the length and degree of con- 

 tractility possessed by the siphons (p,fig- 347). 

 The existence of this muscle, and consequently 

 of siphons, is manifested on the interior of the 

 shell by a posterior sinuous furrow of various 

 depth, and indicating upon a narrow line the 

 point of implantation of the retractor muscle of 

 the siphons. 



In some of the acephalous mollusks the 

 siphons are too large to be received within 

 cover of the shell, in which case the retractor 

 muscle is generally small, inasmuch as it is 

 then of little use (Mya, Glycimeris); but in 

 those species in which the siphons are of mid- 

 dling size, or not so large as to be incapable 

 of entering the shell, the retractile muscle is 

 of considerable size and power (Tellina, 

 Psammobia). 



3. The shell. The lobes of the mantle 

 appear to be the efficient parts in determining 

 the form of the shell, and it is by their thick 

 edges that this covering is in great part secreted. 

 The whole of the Conchiferous acephala,\vithout 

 exception, are included within a bivalve shell, 

 the two parts of which are joined by a point in 

 their upper edge, to which the title of hinge 



has been given by naturalists, and very pro- 

 perly, because it is in truth upon it that the 

 motions of the valves take place. 



General structure. When examined with 

 due attention, the shell is found to be composed 

 of two kinds ot laminae very distinct from one 

 another (a, 6, Jig. 363) ; the one, secreted from 

 within outwards by the edges of the mantle, 

 present themselves under the form of greatly 

 elongated cones, the thick parts of which are 

 turned towards the outer surface (a, c, c, 

 jig. 363) ; the other, in parallel layers, secreted 

 by the central and posterior parts 

 Fig. 363. of the mantle, line the interior of 

 the shell, and in many species at 

 length fill up the cavity of the 

 hooks. These two layers of the 

 shell are frequently found in cer- 

 tain fossil species almost com- 

 pletely separated from one an- 

 other. At other times the inner 

 layer is seen to have been dissolv- 

 ed away, whilst the external one 

 continues without appearing to 

 have undergone any great change. 

 It is in the genera Chama, My- 

 tilus, Pinna, Spondylus, more es- 

 pecially that the two laminae o 

 which a bivalve shell is formed can 

 be studied to greatest advantage, 

 and this study is of importance 

 as leading to a more accurate knowledge of 

 certain fossil genera, in regard to the charac- 

 ter of which some uncertainty has always pre- 

 vailed, by reason of one of the constituent 

 portions of their shell always being found dis- 

 solved, as in Patillus. In some genera the ex- 

 ternal layer is very readily distinguished, from 

 having a fibrous structure (a, a, Jig. 369), a 

 structure observed more especially in the 

 shells of the Pinna family and those of the 

 Malleacea. The two layers of the shell are in 

 the inverse ratio of one another in point of 

 thickness: the external layer, extremely thin 

 towards the hook, increases continually towards 

 the edges, whilst the inner layer, thick at the 

 hook, becomes thinner and thinner as it ap- 

 proaches the edges, around which it is usually 

 exceeded a little by the outer layer. A fact 

 well deserving of attention is this : that the 

 muscular impressions and the whole articular 

 aspect of the hinge are formed in the substance 

 of the inner layer of the shell, and these parts, 

 of so much consequence, do not leave a trace 

 upon the external layer when this alone is pre- 

 served. It is only from having neglected to study 

 the structure of the shell with sufficient attention 

 that naturalists have found themselves at a loss 

 to discover the true characters of certain fossil 

 genera, as Podopsis, Spherulites, which, in 

 consequence of their position in porous chalky 

 beds, never occur with more than the outer 

 layer of their shell in a good state of preser- 

 vation. 



The hinge. The part of the edge of a shell 

 by which the two valves are conjoined, is, as 

 we have already had occasion to state, deno- 

 minated the hinge. This part is entirely formed 

 by the inner layer of the shell. The part of 



