34 



been evolved upwards ; and on the other, the Heteromya and 

 the Monomya have degenerated. 



The Sinupalliata have remained Isomya, both adductors 

 being well developed, but the posterior parts of their mantle 

 lobes have become prolonged and united to form a pair 

 of siphons through which the water flows into and out 

 of the pallial cavity, and the line of attachment of the 

 mantle to the shell has been inflected so as to form a 

 sinus into which the siphons may be more or less completely 

 withdrawn. 



In the other series we have, starting from one of the 

 primitive Integropalliate Isomya, the anterior adductor 

 muscle becoming more and more reduced so as to produce 

 first the Heteromya, including such forms as Mytilus, where 

 the muscles are very unequal in size, and then the Monomya 

 (Ostrea, Anomia, Pecten, &c.), where the anterior adductor 

 is entirely absent. That the Monomya have really degene- 

 rated from Isomya is very clearly proved by Huxley's 

 discovery that the oyster {Ostrea) when very young has both 

 adductors well developed, but loses the anterior one later on, 

 thus recapitulating in its development the ancestral history. 

 The various groups of Lamellibranchiata differ from one 

 another also in the structure of the gill-lamellae, which may 

 become very complex, and in the special development of the 

 pallial siphons and the foot. 



In the second ancestral line, starting from the Archi- 

 Mollusc, the prostomial region has been retained and 

 developed into a well-marked head with cephalic sense 

 organs, and a special and very remarkable organ, the 

 odontophore, has been developed in connection with the 

 stomodgeum. This organ consists essentially of a band, 

 covered by transverse rows of chitinous teeth, stretched over 

 a cartilaginous mass placed on the floor of the buccal cavity, 

 and capable of being protracted and retracted by special 



