182 



HOMOLOGY 



open to the outside on the basal segment of the fourteenth ap- 

 pendage (Fig. 68, 14). From this opening the spermatozoa, 

 packed together in bundles called spermatophores, are caught 

 by the tubular exopodites of the fifteenth pair of appendages, 

 and placed on the genital groove of the female during copulation. 

 The ovaries are similar in general shape and in position, but 

 are bright orange in color, and the female gonoduct or oviduct, 

 while it originates in the same relative position, opens to the 

 outside on the basal segment of the twelfth appendage. The 

 eggs are fertilized as they pass out, and are covered with a 

 gelatinous mucus by which they stick to the hairs bordering the 

 swimmerets or abdominal appendages. Thousands of them 



become thus attached, to 

 be swayed back and forth 

 by the movements of the 

 swimmerets during the 

 early stages of develop- 

 ment. 



DEVELOPMENT AND 

 METAMORPHOSIS.- -The 

 development of the earth- 

 worm, as of Hydra, begins 



FIG. 77. A typical nauphus larva of the cop- . . 



epods with three pairs of appendages. With the division of the 



egg cell into two cells or 



blastomeres, each with one-half of the fertilization nucleus. 

 Development of the lobster begins with the division of the 

 nucleus, without division of the egg substance. The second, 

 third, etc., up to the eighth division are the same, a multinu- 

 cleated cell resulting, in which the nuclei arrange themselves 

 around the periphery of the egg. Then the outer zone of proto- 

 plasm divides around each nucleus, the cleavage planes passing 

 radially toward the egg center (Fig. 76). This type of cleavage 

 is characteristic of the arthropods, and is called meroblastic, as 

 opposed to holoblastic. The yolk is collected in the center of 

 the egg, which for this reason is called centrolecithal. 



Metamorphosis. In the more generalized types of Crustacea 

 this method of cleavage leads to the formation of a free living 



