THE SKIN. 



existence of the latter thus predicates that of the former. The 

 siphonal tube is sometimes greatly prolonged, and is then fre- 

 quently covered for. most or all its length by a prolongation of 

 the aperture, which is technically known as the canal of the shell. 

 The canal in Murex and Fusus is extremely long, at least in the 

 typical species. Mollusks of which the shells are furnished with 

 a canal or anterior notch are called siphonostomata, the first 

 great division of the prosobranchiates. The siphon is principally 

 confined to predatory or carnivorous mollusks. The second 

 great division, termed holostomata. have rounded apertures ; 

 consequently no siphon but simply an opening for respiration. 

 They are vegetable feeders usually (Natica is a remarkable ex- 

 ception), and close the aperture of the shell completely by their 

 operculum. 



At the posterior left border of the mantle, behind the branchiae, 

 is sometimes an opening from which a small siphon extends back- 

 wards, and -when it is present, it forms a notch in the posterior 

 part of the shell, as in (\ypraea and Conus, or a canal as in Ovula, 

 or frequently it only forms a callosity on the upper part of the 

 columella, close to its junction with the posterior part of the 

 aperture margin. Probably this siphonal opening is for the exit 

 of the water that has entered by the branchial opening. In 

 many of the siphonostomata it is not present. 



The mantle border can be freely withdrawn within the whorl, 

 as it is not united to the shell at any point. It is frequently 

 prolonged into digitations, or exhibits prominences or invagina- 

 tions, all of which develop similar features on the shell ; thus 

 giving rise to the fingers of Pteroceras, the spines of Murex, etc. 

 Occasionally, however, processes of the mantle do not secrete 

 shelly coverings : Cerithium and the oriental Melanians, for in- 

 stance, have delicately digitated mantle margins, these digita- 

 tions forming no secretion, and sometimes thrown back over the 

 shell. 



The mantle is occasionally largely developed into side lobes, 

 which in Marginella and Cypraea are so extended as to be thrown 

 up over the external surface of the shell, nearly or completely 

 covering it. In such shells an epidermis is not present. The 

 mantle lobes of Cypraea are beset with numerous papillae, which 

 seem to partake the function of tentacles as tactile organs. In 



