584 Transdctions. 



\ ill. in diameter. The ojfifice is bounded above by the mantle-edge, and 

 below by the body-wall, which is here differentiated to form a kind of 

 muscular valve (Plate XXVIII, figs. 1,2; tm). To close the aperture this is 

 brought into contact with the mantle-edge by means of its muscles. The 

 creature thus has the power of opening and closing this respiratory orifice. 

 The anus opens into this orifice on the lower side (Plate XXIX, figs. 2, 3). 



The single genital pore also opens on the right side, at the junction 

 of head and body (Plate XXVIII, figs. 2, 3 ; (jpY 



Mantle-edfjc. Plate XXVIII, fig. 2. 

 The free edge of the mantle extendi all round the upper edge of the 

 creature, just inside the shell (Plate XXVIII, fig. 2 ; me). This is th? lower 

 margin of the mantle, and normally it lies close to the shell all round and 

 nearly flush with its margin. Its edge resembles in colour and form that 

 of the shell. On irritation the pallial muscles draw it up into the shell 

 with the body. In it are numerous blood-vessels, and it is probably respi- 

 ratory. Mucous glands like those of the body-wall are present in this part 

 of the mantle. 



Morphology and Histology. 

 Pallial Organs. Plate XXVIII. fig. 7 ; Plate XXIX, fig. 2. 



The shell is attached to the creature by means of the pallial muscles, 

 which extend round the latero-dorsal margin of the body, except at the 

 siphonal notch, thus forming a ring broken only at this point, as is indicated 

 by the muscle-impressions on the shell mentioned above (Plate XXVIII, fig. 7 ; 

 ym). At the siphonal notch and over the rest of the dorsal surface the 

 tissues are quite free from the shell. The pallial muscles are thickest near 

 the siphonal notch. 



On dissecting the muscles away from the shell, the latter may be re- 

 moved, and the mantle, which extends over the whole dorsal surface, is ex- 

 posed (Plate XXVIII, fig. 7). It encloses between itself and the dorsal body- 

 wall a large pallial chamber, which extends out as far as the pallial 

 muscles, by which the mantle is attached to the body. In the region of 

 the heart, however, the roof of the pallial cavity is attached to the dorsal 

 wall of the pericardium. This chamber commmiicates with the exterior by 

 the respiratory orifice opening on the right side — a comparatively small 

 branchial aperture — a contrast to the extensive apertures of a large section 

 of the Gastropods : in this feature Siphonaria resembles the Pulmonates. 

 The pallial organs can be seen by transparency. Perhaps the most interest- 

 ing of these organs is the lung ; it is formed by about the anterior third 

 of the mantle, which is very vascular (Plate XXVIII, fig. 7 ; I). Just behmd 

 this, on the left side of the chamber, can be seen the heart, lying in the 

 pericardium ; this I shall include in the account of the vascular system. 

 The kidney is attached to the walls of the second third of the chamber, 

 and immediately behind this the uill is seen forming a semicircular band 

 round the posterior region (Plate XXVIII, fig. 7 : PLate XXIX, fig. 2 ; g). 



The lung is similar to that of the Pulmonates. From a vessel running 

 round the anterior margin of the mantle numerous vessels are given off' 

 which run backwards in the mantle ; some of these give rise to two and 

 even three rami. A corresponding set of vessels take their origin among 

 the terminations of these, and convey the blood to the pulmonary vein, 

 which runs along the posterior edge of the lung to the heart (Plate XXVIII, 

 fig. 7, ef.v, af.v; Plate XXIX, fig. 2). 



