BEHAVIOUR OF SHORE ANIMALS 229 



complexity. In their simplest form they are merely groups 

 of pigmented cells situated on those parts of the body which 

 can be protruded from the shell, that is to say, on the siphons 

 (Pelseneer, loc. cit.). 



In some forms, a group of pigmented cells come into 

 closer association, forming a compound or faceted eye, 

 e.g. Area and Pectunculus . In Pecten the pallial eyes are 

 much more complex, and their structure is superficially 

 not unlike that of the vertebrate eye. They appear as 

 shining black beads at the edges of the mantle and are very 

 beautiful. The supposedly close resemblance between the 

 eye of Pecten and that of Vertebrates has provided material 

 for a considerable amount of discussion. According to 

 Dakin (1912), however, any hypothesis founded on such 

 a conclusion must necessarily be unsound, since there is 

 no real structural or developmental resemblance between the 

 two. The only feature common to the two types of eye is 

 an inverted retina and this is by no means rare in the animal 

 kingdom. These pallial eyes are with one exception (the 

 near relative of a marine form) entirely absent from fresh- 

 water bivalves and are most abundant in genera living between 

 tide-marks, so that it has been suggested that they have arisen 

 in response to the special needs of this area. Forms such 

 as Solen, Mya, and Mactra, for instance, wJiich live between 

 tide-marks and are in the habit of protruding their siphons 

 and sometimes part of their shells, are " manifestly very much 

 at the mercy of their watchful enemies the gulls, and anything 

 which would enable them to apprehend the approach of 

 their enemies would be greatly to their advantage." Again, 

 Pectefi being a relatively active form and given to darting 

 rapidly through the water, it is thought that the eyes may be 

 of assistance to the animal in directing its movements so 

 as to escape its enemies (Cooke, 1895). 



REACTIONS OF SHORE ANIMALS TO THEIR ENVIRONMENT 



The environmental factors which have had most effect 

 upon the behaviour of shore animals are : (i) light, 



