84 RESPIRATION. 



and rudimentary ; sometimes, however, as in Turbo and Phasi- 

 anella, the two gills are brought close together, so as to appear 

 almost as one. In the non-spiral shells, Fissurella, Parmo- 

 phorus, 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 jDlaced. At 

 the same place also the rectum enters and passes forwards on 

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

 angles, the gills are placed, with the laminse free and extending 

 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 passages and 

 between these sexual organs and the rectum there is frequently 

 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 respiratory cavity 

 between the intestine and branchiae is embraced by the frequently 

 large mucous gland, and between it and the intestine there is 

 frequently a special color gland (Purpura, Murex). 



The respiratory cavity has its external opening on the back of 

 the neck, under the mantle-border, which is here contracted to 

 form a rounded hole. Sometimes the walls of this opening are 

 produced into a canal or siphon (xlix, 14), and this difi'erence 

 is one of much importance, agreeing with important modifications 

 of the shell, and (excepting Natica) with diff'erence 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 (Ixii, 21). 



Although there is no true siphon in the holostomata or 

 herbivorous prosobranchiates, one of the neck-lappets is some- 

 times curled over, so as to form an analogous organ, as in 

 Paludiua and AmpuUaria. The in-coming and out-going currents 

 in the branchial chamber are separated by a valve-like fringe 

 developed from the neck-lappet. In Fissurella, Haliotis (and 

 the Scaphopoda) the respired current is still more efl'ectually 



