RESPIRATION. 41 



are brought close together, so as to appear almost as one. In 

 the non-spiral shells, Fissurella. Parmophorus, and in Haliotis 

 the gills are symmetrical and both well developed. 



The form of the depressed respiratory cavity is triangular, in 

 the hinder angles of which the heart and kidney are placed. 

 At the same place also the rectum enters and passes forwards 

 011 its right side. At the left side of the same, but attached to 

 its angles, the gills are placed, with the lamina' free and extend- 

 ing into the cavity simple or double, with their basal position in 

 relation with the heart. 



On the floor of the cavity, at the right side, by the rectum, 

 lies the vagina or the ciliated furrow of the seminal passage 

 and between these sexual organs and the rectum there is fre- 

 quently pushed the- tubular, inflated, excretory duct of the 

 kidney ; above on the rectum sometimes lies the prolonged anal 

 gland with its opening in front of the anus, so that the openings 

 of the anus, anal glands, kidney and sexual organs are arranged 

 close together in the above order from without inwards at the 

 right anterior side of the cavity. The covering of the respira- 

 tor} r cavity between the intestine and branchiae is embraced by 

 the frequently large mucus gland, and between it and the intes- 

 tine there is frequently a special color gland. ( Purpura, Murex.) 



The respiratory cavity has its external opening on the left 

 side of the body, under the mantle border, which is here con- 

 tracted to form a rounded hole. Sometimes the walls of this 

 opening are produced into a canal or siphon, and this difference 

 is one of much importance, being co-extensive, with important 

 modifications of the shell, and i excepting Natica) with difference 

 of food. Thus the Siphonostomata, as already stated, have the 

 shell terminating in a notch or canal below, and are carnivorous, 

 whilst the animals with a sessile respiratory opening belong to the 

 Holostomata, the shells with rounded apertures, and ( with the 

 exception of Natica) vegetable feeders. In some of the Murices 

 the canal of the shell is very long, but where the canal is short 

 or the aperture simply notched below, it by no means follows 

 that the siphon is short ; on the contrary, in Cassis and Dolium 

 it attains an extraordinary length, and is reflected over the back 

 of the shell, so that the borders of its wall become dorsal. 

 6 



