ZOOLOGY FOR MEDICAL STUDENTS CHAP. 



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



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



neiicement 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- 



dbnp-. 77. In the typical Cuttlefishes there opens into the intestine 



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. 



Tin. 1 body-cavity of the mollusc like that of the arthropod is haemo- 



. in its nature -a large part of the blood system having degenerated 



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



pongework the remnants of the vessel walls and other solid 



tissue 



The coelome has become reduced to two relatively small cavities 

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

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

 genital coelome the cavity of the ovary or testis. 



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



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



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



nsive modification. Those originally belonging to the genital 



me ha\e 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- 



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



its original connexion with the pericardiac cavity and becomes 



'tital duct. In the Pelecypoda it would appear, judging 



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



ling trom the pericardiac cavity once served as genital ducts. 



In the majority of existing I'elecypods, however, the opening from 



or testis lias been shifted outwards along the course of the 



nephridium until it now lies outside the nephridium altogether on the 



'irface of the body. 



molliiM-im nervous system shows a particularly characteristic 

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

 lia clustered round the oesophagus the cerebral 



