XII CHARACTERISTICS OF ABYSSAL MOLLUSCA 375 



occur in the tropical Murex and Purpura^ are not found in deep- 

 sea species. But the ornamentation is frequently elaborate, and 

 the sculpture rich and varied. There is an especial tendency 

 towards strings of bead-like knobs, revolving striae, and delicate 

 transverse waves, the sculpture being in many cases of a 

 character which tends to strengthen the structure of the shell, 

 like the ridges in corrugated iron. 



A remarkable feature in some deep-sea Mollusca is their 

 singular resemblance, in shape, and particularly in the possession 

 of a strong green periostracum, to some of our common fresh- 

 water species. According to Dr. Dall, the cause of this phe- 

 nomenon is the same in both cases. The fresh-water Mollusca 

 secrete a strong periostracum, in order to protect the shell against 

 the corrosive influence of the carbonic acid gas with which the 

 water is surcharged. The shells of deep-sea Mollusca, living, as 

 they do, in water probably undisturbed by currents of any kind, 

 have to protect themselves against the same eroding influence, 

 and do so in the same way.^ 



Mollusca which live exclusively on algae and other forms of 

 plant life are almost entirely wanting in the great depths, where 

 vegetation is probably unknown. The struggle for existence 

 must be much less keen than in the thickly populated shallows, 

 where vicissitudes of every kind occur. The absence of rapid 

 motion of water must obliterate many of those mechanical effects 

 which tend to produce modifying influences upon the animals 

 affected. In the absence of circumstances tending to cause 

 variation, in the unbroken monotony of their surroundings, 

 species must, one would think, preserve a marked uniformity 

 over an exceedingly wide area of range. 



Vegetable food being wanting, those genera which in shal- 

 lower waters never taste flesh, are compelled to become carnivo- 

 rous. Characteristic of the great depths are very remarkable forms 

 of Trochidae, in whose stomachs have been found the remains of 

 Corallines and Foraminifera. According to Dr. Dall, the results 

 of this diet show themselves in the greatly increased space 

 occupied by the intestine, in the diminution, as regards size, of 

 the masticatory organs, the teeth and jaws, and also in the pro- 

 longation of the anal end of the intestine into a free tube, which 

 carries away the excreta in such a way that they do not foul the 



1 W. H. Dall, Proc. Biol. Soc. Washington, v. p. 1 f. 



