234 Dzvellers of the Sea and Shore 



hexagonal, rectangular. On the other hand, where the 

 facets are mere circular units unmethodically arranged, 

 such as is found in the so-called "silver fish" and certain 

 isopods, quickness of perception is lacking. The eyes 

 of the horseshoe crab are of this type. Just what is 

 the connection between a compound eye composed of 

 a loose aggregation of units and its want of acuteness 

 is hard to determine. Nor is it easy to say why Lim- 

 ulus, which seems to be neither crustacean nor Insect, 

 yet possessing two pairs of eyes, one pair actually con- 

 sisting of nearly two thousand smaller eyes, does not 

 make better use of its endowment. Nevertheless, if 

 one were to hazard the guess that here we see the 

 compound eye of higher animals in evolution, he would 

 probably be not far from the truth. For it is a sig- 

 nificant fact that all creatures distinguished by what 

 appears to be this primitive form of compound eye are 

 creatures of a very ancient type; they differ but little 

 to-day from their fossils which were formed millions 

 of years ago. Down through the ages which saw count- 

 less other forms appear and then vanish away, they and 

 our Limulus have survived practically unaltered in 

 form. 



The eggs spawned by a large female horseshoe crab 

 number upward of ten thousand, enough almost to fill 

 a half-pint jar. They are spherical bodies measuring 

 about one twelfth of an inch in diameter. In color they 

 are green. Under the microscope will be seen in each 

 egg a large, smooth granular yolk nearly filling the 

 tough, transparent membrane that acts as a shell. The 

 rate of development of the embryo depends on tem- 

 perature and, to some extent, light. In a cool place and 



