THE SKIN. 13 



mantle. Mollusks are even able to secrete shelly matter to 

 provide against threatening dangers from the boring of other 

 animals into their shell. A curious example of shell secretion 

 by the visceral mantle occurs in a cone belonging to the cabinet 

 of the late Dr. Gray. A section of this shell has been made, 

 showing holes bored into the spire end by lithodomi and the 

 repeated walls erected by the animal across the ends of the 

 whorls to protect itself against the ravages of its insidious 

 enemies (i, 8). 



The mantle border by means of its sphincter muscles embraces 

 the body closely, thus closing the mantle cavity except at one 

 point, where a small opening allows the ingress and egress of 

 water for respiration. This respiratory opening is a semicircular 

 notch, formed by muscles, and is sometimes prolonged on its 

 dorsal wall into a half-closed tube or respiratory siphon (xvi, 89), 

 which, when present, assists by the phases of its development 

 in the classification of the mollusca. This siphon usually forms 

 an anterior notch in the shell near the margin of the columella 

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

 The siphonal tube is sometimes greatly prolonged, and is then 

 frequently 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 prosobranchiate gastropods. The siphon 

 is principally confined to predatory or carnivorous mollusks. 

 In the second great division, termed holostomata, the shells have 

 rounded apertures ; consequently no siphon but simply an opening 

 for respiration. They are vegetable feeders usually (Natica is a 

 remarkable exception), and close the aperture of the shell com- 

 pletely by an 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 Cypraea and Conns, or a canal as in Ovula, 

 (Ixi), 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. This siphonal opening serves for the exit 

 of the water that has entered by the branchial opening. 



The mantle border can usually be freely withdrawn within the 

 whorl, as it is not united to the shell at any point. It is fre- 

 quently prolonged into digitations, or exhibits prominences or 

 invaginations, 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 (iii, 45). Cerithium and the oriental 



