BEHAVIOUR OF SHORE ANIMALS 227 



have an olfactory function. If crabs and lobsters are observed 

 in an aquarium, these antennules are often to be seen in 

 rapid vibration, and this is especially the case when the animals 

 have been moved from one place to another. The antennse 

 proper are obviously tactile in function. 



The eyes of Crustacea are of the w^ell-known compound 

 type, thought, until quite recently, to give only " mosaic " 

 vision. They are either stalked, as in crabs, shrimps, and 

 lobsters, or sessile, as in Isopods and Amphipods. 



The appearance in higher Crustacea of a perfected 

 visual (image-forming) organ has meant a great addition 

 to the number and variety of elements capable of affording 

 sensations (colour, form, etc.), and of entering into com- 

 bination with one another to bring about a particular 

 reaction or series of reactions. The fact that such combina- 

 tions are numerous, complex, and flexible (capable of being 

 modified in a new environment), means that the behaviour 

 tends to become less predictable and more " intelligent." 



An important point made by Bohn is that among a number 

 of sensations acting in conjunction to produce a particular 

 reaction, one may gradually acquire predominance and 

 eventually become so important as to be sufficient in itself to 

 produce the reaction. There is thus a process of secondary 

 simplification. But while this is an advantage in some 

 cases, in others it may lead the animal astray. Thus, 

 hermit crabs when seeking a Gasteropod shell tend to 

 explore any round object, whether this possesses an aperture, 

 i.e. is of use to their purpose, or not. That is to say, among 

 the many variable features which distinguish shells the 

 curved nature of the surface is the most essential and has 

 so become the trigger which releases a whole series of 

 reactions quite independently of any other features which 

 may be present. 



Molluscs. — The details of the nervous organisation of 

 this phylum vary considerably in the different groups. 

 Generally speaking, a series of paired cords innervate the 

 chief organs (foot, mouth, and viscera) and these are linked 

 in the head region by circular commissures, and elsewhere 



