70 



Arenicolidae 



single statolith throughout life. In A. ecaudata and Iranchialis 

 (Fig. 41) there is at first only one statolith. Later many others are 

 formed, but the original one remains conspicuous by reason of its 

 larger size. 



The statocysts of A. assimilis attain a much larger size than those 

 of any other species : indeed, they are the largest known in the 

 Polychaeta. There is some difference in the size of these organs in 

 similar specimens of the same species from different localities. 

 Fig. 38 A, B, which represent the statocysts of similar specimens of 

 A. assimilis var. affinis, from Otago and the Falkland Islands 

 respectively, illustrate an extreme instance of local variation in the 

 size of these organs. 



The eyes are of simple structure. Each is formed of a lens 



Fig. 40. A. cristata. Statolith, and 

 cuticular lining of statocyst. 



Fig. 41. A. branchialis. Stato- 

 liths, and cuticular lining of 

 statocyst. 



and a cup-shaped mass, about 8 to 20.//, in diameter, composed of 

 reddish-brown pigment-spherules (PI. X, Fig. 25). In the early 

 stage of larval life the eyes are actively functional, and during the 

 first two days after hatching the larvae are phototropic ; but by the 

 third day this preference for light is lost. The eyes soon sink below 

 the epidermis, and become imbedded in the masses of ganglion-cells 

 which form the dorsal and anterior portion of the brain. With the 

 increase in the amount of pigment in the prostomial epithelium the 

 eyes become difficult to find, and are finally indistinguishable. It 

 seems unlikely that in adult specimens the eyes' can have any 

 functional importance. 1 



1 Arenicola is sensitive to light, resembling earthworms in its reaction to 

 this stimulus, and, as in them, the response may be due to the stimulation of 

 the sense-cells in the epidermis, especially of the anterior end ; it is unlikely 

 that it is due to specific action through the eyes, for the reasons given above. 



