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 Astacics, to 

 the formation of endo- 

 derm by the ingrowth 

 of a solid plug of cells 

 (Arthrostraca, &c.). 



Compared with As- 

 tacus, however, the most 

 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 

 {Nehalia, 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. 



Fig. 91. — Zoa:a of common Shore Crab 

 (Carcintis nuciias). (After Faxon.) 

 The appendages are numbered. 



