GENERAL NOTES ON CRUSTACEANS. 



281 



diverse. The spermatozoa are usually exceptional in being 

 very slightly motile. Some appendages are often modified 

 for copulation or for carrying the eggs. 



Development. The ova of most Crustacea show con- 

 siderable similarity to those of Astacus, and the segmen- 

 tation is typically of the kind already described. But 

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



indeed, for Arthropod 

 development, it is pos- 

 sible, within the limits 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 gastru- 

 lation is usually much 

 disguised, we find all 

 cases from an invagina- 

 tion of the simplest em- 

 bolic type (Lucifer), and 

 through the condition 

 described for Astacus^ to 

 the formation of endo- 

 derm by the ingrowth 

 of a solid plug of cells 

 (Arthrostraca, &c.). 



FIG. 91Zcxea of common Shore Crab Compared with As- 

 (Carcimts mcenas}. (After FAXON.) tacus, however, the most 

 The appendages are numbered. important point we have 



to notice is the frequent 



occurrence of a very striking metamorphosis in the life 

 history. In other words, the larva hatched from the egg 

 is rarely like the parent, and only acquires the adult char- 

 acters after a series of profound changes. In some cases 

 (Nebalia, Mysis) a metamorphosis takes place within the 

 egg-cases, and in the few forms in which development seems 

 to be direct, slight traces of metamorphosis are found. 



