276 ZOOLOGY FOR MEDICAL STUDENTS chap. 



liver. Typically in the Pelecypoda and in some of the more primitive 

 Gasteropoda there is formed in a glandular recess of the stomach, or 

 commencement of the intestine, a curious glassy-looking body^ — the 

 crystalline style — apparently a condensed mass of digestive ferment and 

 of practical interest as being the haunt of the large Spirochaetes men- 

 tioned on p. 77. In the typical Cuttlefishes there opens into the intestine 

 close to the anus a very large gland — the ink-sac — which secretes the 

 dark brown pigment known to artists as sepia. When alarmed the 

 Cuttlefish allows some of the secretion to pass into the mantle-cavity, 

 whence it is blown out through the siphon as an opaque cloud behind 

 which the creature is able to escape unseen. 



The body-cavity of the mollusc like that of the arthropod is haemo- 

 coelic in its nature — a large part of the blood system having degenerated 

 into a system of irregular spaces filled with blood and traversed by 

 a sparse spongework — the remnants of the vessel walls and other solid 

 tissues. 



The coelome has become reduced to two relatively small cavities — 

 the pericardiac cavity which surrounds the heart and in the more primitive 

 molluscs is still traversed by a small part of the ihtestine, and the 

 genital coelome — the cavity of the ovary or testis. 



There is some reason to believe that primitively each of these cavities 

 communicated with the exterior by a pair of tubular nephridia, but in 

 the course of the evolution of the MoUusca these nephridia have under- 

 gone extensive modification. Those originally belonging to the genital 

 coelome have completely disappeared except in Nautilus and in some 

 of the more primitive Gasteropods (Chiton), in which they function as 

 genital ducts. On the other hand the right nephridium associated with 

 the pericardiac coelome establishes a connexion with the ovary or testis 

 in some of the lower Gasteropods and serves for the exit of the repro- 

 ductive cells, while in the more highly evolved members of the group it 

 loses its original connexion with the pericardiac cavity and becomes 

 simply the genital duct. In the Pelecypoda it would appear, judging 

 from the more primitive forms (Protobranchia), that both nephridia 

 leading from the pericardiac cavity once, served as genital ducts. 

 In the majority of existing Pelecypods, however, the opening from 

 the ovary or testis has been shifted outwards along the course of the 

 nephridium until it now lies outside the nephridium altogether on the 

 external surface of the body. 



The molluscan nervous system shows a particularly characteristic 

 arrangement in the ordinary Gasteropods. Here (Fig. 114, A) there are 

 three pairs of large ganglia clustered round the oesophagus — the cerebral 



