DEVELOPMENT OF CRUSTACEA 357 



shells, etc., and it is often complicated by the simultaneous 

 existence of " pigmy " complemental males. When 

 separate, the two sexes are often very diverse. The sper- 

 matozoa are often exceptional in being very slightly 

 motile. Some appendages are often modified for copula- 

 tion or for carrying the eggs. 



Development. — The ova of most Crustacea show con- 

 siderable similarity to those of Astacus, and the segmenta- 

 tion is typically of the kind already described. But while 



Fig. 200. — Partial peripheral segmentation of the ovum — 

 characteristic of Arthropods. 



A. A vertical section of the segmented ovum, showing some rather larger segmenta- 



tion cells, the macromeres [MA.], and some rather smaller cells, the micromeres 

 (MI.). The nuclei of the cells are indicated by dark spots. BL., Blastomeres, 

 the term applied to the segmentation-cells in general, whether macromeres 

 or micromeres, or all equal. 



B. A surface view of the same ovum, showing macromeres (MA.) and micromeres 



(MI.). The surface view of an ovum showing this type of segmentation is 

 like the fruit of rasp or bramble. 



this is the most typical case for Crustacean, and, indeed, 

 for Arthropod development, it is possible, within the Hmits 

 of the class Crustacea, to trace out a complete series, in 

 which the first term is a segmentation of the complete 

 and equal type, like that of a worm, and the last the purely 

 peripheral. In the same way, though gastrulation is 

 usually much disguised, there are many modes, from 

 an invagination of the simplest embolic type (Lucifer), and 

 through the condition described for Astacus, to the forma- 

 tion of endoderm by the ingrowth of a solid plug of cells. 



