Decapod Crustacea of the North Atlantic. 197 



of the species of GJyphocrangon and Pontophilus ahyssi tlirough 

 a stage like the ejes of Calocaris, which are practically 

 sessile, have lost all of the normal visual elements, and have 

 only colourless pigment, but still present a large flattened 

 transparent cornea at the anterior margin of the carapace. 



It is interesting to note that the highly modified eyes of 

 Pentacheles are found in a well-defined group, all the species 

 of which have probably been inhabitants of deep water 

 for considerable geological periods ; while the equally deep- 

 water species with less modified or obsolescent eyes are much 

 more closely allied to shallow-water species, from wliose an- 

 cestors they may have been derived in comparatively recent 

 times. 



The large size and small number of the eggs is a very 

 marked characteristic of many deep-sea Decapoda. The eggs 

 are extraordinarily large in several species of Munidopsisj 

 Olyphocrangon^ and Bythocaris^ and in Elasmonotus inermis, 

 Sabinea princejps^ and Pasiphae princeps. But the largest 

 Crustacean ^g^ which I have seen is that of the little shrimp 

 Parapasiphae sulcatifronSy which carries only from fifteen to 

 twenty eggs, each of which is more than 4 millim. in diameter, 

 and approximately equal to a hundredth of the bulk of the 

 animal producing it. My suggestion (Amer. Journ. Sci. xxviii. 

 p. 56, 1884) that the great size of the eggs in the deep-water 

 JJecapoda was probably accompanied by an abbreviated 

 metamorphosis within the egg, thus producing young of 

 large size and in an advanced stage of development, specially 

 fitting them to live under conditions similar to those envi- 

 roning the adults, has already been proved true by Prof. G. 

 O. Sars in the case of Bythocaris leucopis, in which the young 

 are in a stage essentially like the adult before leaving the egg. 

 Although the great size of the eggs is highly characteristic 

 of many deep-water species, it is by no means characteristic 

 of all ; and, as the following Table of measurements shows, 

 the size of the eggs has no definite relation to the bathy me- 

 trical habitat and is often very difierent in closely allied 

 species, even when both are inhabitants of deep water. For 

 example, the eggs of Acanthephyra gracilis are very large, 

 while those of A. hrevirostris and A. Agassizii are normally 

 small, and those of Pontophilus ahyssi are fully as small as 

 in the comparatively shallow-water species of the genus, and 

 much smaller than those of many shallow-water species of 

 Crangonidaj. 



For the purpose of comparing the size of the eggs of deep- 

 and shallow- water species, measurements of the eggs of a 

 number of species of Decapoda, and in some cases the number, 

 or approximate number, carried by an individual, are given in 



