72 THE NERVOUS SYSTEM OF MOLLUSKS. 



the present clay is also very wide.- Like the Tunicata, they 

 are also headless organisms, and lead a sedentary existence, 

 attached to rock or stone either by a pedicle or by one 

 division of their bivalve shells. The mouth is unprovided 

 with any appendages for grasping food — nutritive par- 

 ticles being again brought to it by means of ciliary currents. 

 Numerous muscles exist which connect the valves of the 

 shell to one another, and with the enclosed animal. 



Though the visceral organization of the Brachiopods 

 is somewhat complex, no definite Sense Organs have 

 yet been detected in any of them. The nervous system of 

 these sedentary animals, moreover, comprises nothing an- 

 swering to a ' brain ' as it is ordinarily constituted — though 

 ganglia exist aroimd the oesophagus which must receive 

 afferent impressions of some kind, and from which 

 branches proceed to the various muscles and viscera of 

 the body. 



Such low sensory endowments would be wholly 

 incompatible with that degree of visceral complexity of 

 organization which the Brachiopods possess, had it not 

 been for the fact that these animals lead a passive exist- 

 ence in respect to quest of food. The absence of sense- 

 organs and of a brain is, indeed, only compatible with 

 such a semi- vegetative existence. 



The Lamellibranchs, or ordinary headless bivalve 

 Moilusks, also include some representatives — such as the 

 Oyster and its allies — which lead a sedentary life. The 

 valves of the shell in Lamellilranchs generally are lateral, 

 instead of being dorsal and ventral as amongst the 

 curious Brachiopods above referred to. 



The mouth of the Oyster is surrounded by four labial 

 processes whose functions are not very definitely known. 

 It presents no other ai)pendages of any kind in the 

 neighbourhood of the mouth, and, as in the two types of 



