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" - 



{ephippiuvt) is formed, which is cast -c 



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 



of winter -ecrg's enclosed in each case 



depends upon the species, varying from 



one to several. 



In Fork- Footed Crustaceans {Cope- 

 poda) 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 Fig. 

 easily-obtained example is furnished by 

 Cyclops (fig. 886) which often abounds 

 in ponds. The two ovoid egg -bags 

 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' 

 pedia) are the stalked Ship Barnacle {Lepas) and the stalkless 

 Acorn Barnacle [Balamis), which is found in great numbers 

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



— Water-Flea [Daphnia), with Eggs 

 in Brood-pouch; enlarged 



1-7, Appendages; d, digestive gland ; i, in- 

 testine: k, heart; k, kidney; b, brain with 

 larval eye (the black spot); op, optic ganglion 

 and compound eye; ov, ovary; od, opening 

 of oviduct ; p, brood-pouch containing two 

 developing eggs. 



