248 GEOLOGICAL BIOLOGY. 



development of its organs is to compare it with the radiate 

 type of the Coelenterata — to suppose a Coelenterate with 

 its row of tentacles, specialized in function and differentiated 

 in structure, so that, first, the contractile function is on the 

 one side, and muscular tissue is developed in place of numer- 

 ous tentacles, thus forming a mass of fleshy tissue contracting 

 in two directions and accomplishing locomotion ; while the 

 correlative function of sensibility is specialized on the other 

 side of the mouth into two tentacles, with, in some of the 

 higher types, distinct and well elaborated eyes for sensation. 

 In the Coelenterata the tentacles are both sensitive and con- 

 tractile, they are multiple, and muscular tissue and nervous 

 tissue are not fully differentiated. T\\& foot of the Mollusca 

 may be considered as a specialized motor organ, in structure 

 it is differentiated muscular tissue, and marks the side of the 

 body as ventral ; while the tentacles, as of the snail, are spe- 

 cialized sensory organs expressing the differentiation of ner- 

 vous tissue for the special function of sensation, and mark 

 the dorsal side. In the Mollusca and in other branches the 

 gangliation of the nervous coi'd is co-ordinate with supply of 

 nerve-fibres to specialized organs; thus as the sensory organs 

 are more elaborated and specialized there is developed a 

 large nerve-ganglion on the corresponding part of the ner- 

 vous ring, and the foot too has its special nerve-ganglia de- 

 veloped. We find also specialized in the Mollusca tissues 

 for secreting fluids accessory to the digestive function, and the 

 differentiated organ for this purpose may be compared to the 

 liver of vertebrates. The stomach is not, however, highly 

 differentiated from the digestive tract in the lower types. 

 The Mollusca have specialized organs for respiration. These 

 organs — the gills, or mantle fringes — are present in all ; but 

 in the class Lamellibranchia the function is not entirely re- 

 spiratory, but is also partly ingestive, or has to do with bring- 

 ing food to the mouth. 



DiiFerentiation of the Nervous System. — The nervous system 

 is differentiated to correspond to the differentiation of other 

 organs, and in two directions; at an early stage contractility 

 and sensibility were differentiated. Sensibility, then, has 

 two essential relations: sensibility as receptive of impressions 



