CRUSTACEANS 



363 



our observations are made in the summer. Though much smaller 



than their mother they resemble her in appearance, and may be 



seen to play around her for some time. The qualification about 



the time of observation is made because these little creatures 



produce both summer-eggs and winter-eggs, the fate of which is 



somewhat different. The former are thin-shelled and develop 



without fertilization, hatching out during 



the season from which they get their 



name. The winter -eggs are fertilized, 



and, being destined to tide the species 



over the cold part of the year, are 



enclosed in firm, resistant shells. By 



thickening of a part of the parent shell 



a sort of case, known as a " saddle " 



(epkippium) is formed, which is cast 



adrift with a couple of eggs inside it, 



and floats about on the surface of the 



water. The young Water- Fleas hatch 



out the following spring. The number 5 



of winter -eggs enclosed in each case 



depends upon the species, varying from 6 



one to several. 



In Fork- Footed Crustaceans (Cope- 7 

 podd) the eggs, when laid, usually pass 

 into curious little egg-bags, the material 

 for making which is furnished by 

 special cement-glands. A typical and Fi g . 88 S .-w a ter-Fiea (z^*), with E ggs 



11*1 i r 1 i i in Brood-pouch; enlarged 



easily-obtained example is furnished by 



/-* i /r OO/r\ "L * "L C 1_ J 1-7, Appendages; <f, digestive gland ; z', in- 



CyClOpS (fig. SOO) Which Often abOUndS testine; h, heart; k, kidney; b, brain with 



in ponds. The two ovoid egg-bags ^^0"* 



are carried about by the mother till 



the time of hatching. Some Copepods 



possess but one egg-bag, and these structures are differently 



shaped according to the species. The eggs of a few members 



of this group are sheltered in a dorsal brood-pouch, resembling 



that of a Water- Flea. 



The most familiar examples of the group of Barnacles (Cirri- 

 pedid) are the stalked Ship Barnacle (Lepas] and the stalkless 

 Acorn Barnacle (Balanus), which is found in great numbers 

 attached to rocks between tide-marks. In both these cases 



of oviduct; /, brood-pouch containing two 

 developing eggs. 



