LAMELLIBRANCHIATA. 



17 



KIS 



surface of the mantle and on the gills into the mantle and respira- 

 tory chamber. Food materials pass with the water to the labial 

 palps, and so to the mouth. The edges of the mantle lobes <lo 

 not always remain free through their whole extent, but frequently 

 fuse together, first at the hind end, and then gradually forwards. 

 As a result of this fusion, a posterior opening, including in 

 itself the inhalent and exhalent orifices, becomes separated from 

 the anterior opening into the mantle cavity; and, further, the 

 exhalent and inhalent openings become separated from each 

 other by a transverse bridge of tissue. The long anterior opening 

 or foot-cleft) in consequence of the progressive fusion of the mantle 

 edges, often becomes gradually so shortened, that the. foot, which 

 is correspondingly reduced, can 

 scarcely be protruded. In this 

 case, the mantle comes to have 

 the form of a saccular investment 

 with two openings. The further 

 forward .the fusion of the two 

 mantle lobes proceeds, the more 

 marked becomes a peculiar elonga- 

 tion of the posterior mantle region 

 round the inhalent and exhalent 

 openings aK elongation of such a 

 nature that two contractile tubes, 

 or siphons, become formed (fig. 



4 t\c\ \ mi i -L FIG. 499. a, Mactra elliptica, animal with 



499, a). The latter may reach Bhell . ms> cloacal or exhalent siphon . 



such a size that thev can no longer ^ s > branchial or inhalent siphon ; P, foot- 

 , , b, left valve of M. solida; VM, anterior 



be drawn between the posterior adductor muscle ; lor, posterior adductor 

 edges of the gaping valves of 

 the shell. The two siphons often 

 fuse with one another; but the two canals with their openings 

 surrounded by tentacles, remain separate. In the most extreme 

 cases, the siphons are enormously enlarged, and the posterior region 

 of the body is peculiarly elongated and uncovered by the rudimentary 

 shell ; so that the whole animal acquires a vermiform appearance, 

 the shell-bearing anterior region of the body constituting the head 

 (Teredo, fig. 505). 



The mantle and skin consist of a cellular, slimy epidermis, beneath 

 which lies a connective tissue, richly traversed by muscular fibres. 

 The epidermis on the outer surface of the mantle consists of columnar 

 cells ; while on the inner 'surface of the mantle the cells composing 



VOL. II. 2 



muscle; Ml, pallial line; Mb, pallial 

 indentation. 



