ABYSSAL BEXTHAL AND PELAGIAL 



259 



ing to the size of their eyes. Those with large eyes live in the uppermost 

 abyssal, those with smaller eyes in more considerable depths. 



On the other hand, the eyes of many deep-sea animals are especially 

 large, much larger than in their relatives of the upper zone. This is 

 especially true of animals living near the upper limit of the lightless 

 zone (Fig. 68a), which thus correspond to the nocturnal or twilight 

 terrestrial animals with large eyes, such as geckos, owls, and tarsiers. 

 A number of decapod crustaceans exhibit this peculiarity in the abyssal 



Fig. 69. — Deep-sea crustaceans with enlarged eyes, a, a hyperine, Cyrtisoma 

 spinosum; b, an euphausid, Thysanoessa gregaria; c, eye of an euphausid, 

 Stylocheiron mastigophorum in vertical section, in which F is the anterior and S 

 the lateral eye; d, a caridid, Pandalus magnoculus. a, b, and d from the Chal- 

 lenger report; c after Chun. 



benthal (Fig. 69d) . 19 In the pelagial many amphipods (Fig. 69a) , 

 schizopods (Fig. 696), and sergestids agree in the possession of en- 

 larged eyes. Their eyes are divided into a dorsal portion with upward- 

 directed facets which are elongate and little divergent, while the lateral 

 portion has shorter and strongly divergent elements. Such eyes are 

 otherwise unknown in these groups. Numerous fishes of the border zone 

 of the abyssal also have enlarged eyes, for example, Brama longipinnis, 

 Aphanopus carbo, and Regalecus. Certain pelagic cephalopods, for 

 example Chiroteuthis and Pterygioteuthis, have very large eyes; but 

 their habits are insufficiently known to make it certain that this en- 

 largement is in adaptation to abyssal life in these cases. 



Enlargement of the lens alone, instead of enlargement of the whole 

 eye, affords a second type of adaptation to the dim abyssal light, ex- 



