y6 The Irish Naturalist. [ March, 1899. 



corresponding to the four gills), into which the blood flows 

 from the gills (or the pulmonary chamber in the air-breathing 

 Gastropods), and a ventricle which receives it from the 

 auricles and drives it into the body. The venous blood 

 collects in one or more large spaces which supply the 

 respiratory apparatus. 



The nervous system consists of a number of nerve-centres 

 or ganglia joined together by bands and sending out threads 

 to the different organs ; thus there are the cerebral, or else 

 buccal, pedal, and pleural nerve-centres. In the Cephalopods, 

 the highest member of the molluscan group, there is a con- 

 centration of these nerve-centres approaching a veritable 

 brain. 



The eyes of the Mollusca vary greatly, ranging from the 

 simple, pigmented spot with optic nerve, merely susceptible 

 to the impression of light and darkness, to one in which a 

 lens is developed, as in the Dibranchiates, or cuttles. The 

 Gastropoda have usually, and the Cephalopoda invariably, 

 two eyes upon the head, or the head region, but in the first of 

 these groups Chiton has eyes curiously scattered over the 

 surface of the shell, 1 sometimes promiscuously, sometimes in 

 regular rows. Among the Lamellibranchs Pecten has eyes 

 disposed at regular intervals on the two edges of the mantle. 



A curious organ in the Dibranchiates, of protective function, 

 is the ink-sac ; it is connected with the excretory parts and 

 emits a dark fluid which discolours the water and thus 

 enables the animal to escape from its pursuers. The ink-sac, 

 still distended with its desiccated contents, is often found 

 preserved in a fossil state in the Lias (Mesozoic era) of Lyme 

 Regis, Dorsetshire. Dean Buckland, 2 the famous geologist, 

 had a drawing made from this fossil pigment, which an artist 

 to whom it was shown supposed had been executed in sepia 

 obtained from an artist's colourman. 



Lastly, the Scaphopoda are an aberrant group of simple 

 structure, having neither head nor eyes. Their nervous 

 system is fairly developed, but there is no heart, the blood 

 circulating in the spaces contained in the body-cavity. 



1 This brilliant and interesting discovery was made by the late 

 Professor H. N. Moseley, of Oxford. {Quart, fourn. Micro. Sci. % vol. 

 xxv., 1885, p. 37, with plates). 



• Bridgewater Treatises, VI. 



