1918] Essenherg: Distribution of the Polijnoidac 223 



subject to the sudden environmental changes and are less likely to 

 be killed by the temperature or to undergo any variation and abnor- 

 malities. 



There is a general belief that the deep water annelids are without 

 eyes or blind on account of the absence of the light in the great depths. 

 Experiments in the laboratory have proved that absence of light does 

 not produce blindness, but on the other hand by exposing eggs to 

 to 2° temperature, numerous abnormalities and blindness in embryos 

 of the fish Fundulus were produced (Loeb, 1915). Since the corre- 

 sponding low temperature is found in the depths of the ocean it would 

 not be at all unlikely that the low temperature, although it may not 

 change the adult forms which have been carried from warmer areas to 

 the cold waters in the depth, may still affect the second generation— 

 the eggs of the first migrants — and thus produce blindness indirectly. 

 So that the cause of blindness and eyeless condition of the abyssal 

 Polynoidae might be attributed to low temperature rather than to 

 darkness. 



The degree of light intensity may be a cause of modification of 

 the eyes of deep sea animals. It has been found (Brauer, 1901) that 

 in all fish from about 300 meters, the rods only are found in the retina 

 of the eye, a condition which is characteristic also of the eyes of 

 nocturnal animals ; while diurnal animals have both rods and cones. 

 The differences of the pigment of the retina in the deep sea fish signify 

 that their eyes are adapted to nocturnal conditions. Further result 

 of the modification and adaptation is the telescopic eyes of some fish. 

 There are also great numbers of blind fish in the depths of the ocean ; 

 it remains an open question whether the blindness is caused by the 

 low temperature or by the action of some chemicals in the deep sea or 

 by a combination of both. 



That the plasticity is great in Polynoidae is proved by the fact 

 that considerable changes are produced. A remarkable difference 

 is noticeable in the size and shape of different individuals living in 

 different environment. Boreal species as a rule attain a larger size 

 than individuals of the same species occurring in tropical and tem- 

 perate zones. This rule of comparative size within a given species 

 does not hold strictly true as between localized species. The largest 

 known polynoid, Lepidasthenia gigas, is subtropical, inhabiting a 

 limited area on the coast of southern California. It is quite possible, 

 however, that its great size is an unusual development since the 

 animal is an exclusive commensal. 



