72 A BIOLOGY OF CRUSTACEA 



and enclosed in a shell. The hrine shrimp, Artemia salina, is un- 

 usual in that when in favourable conditions it liherates active 

 voting from its hrood pouch. In other conditions Artemia lays eggs, 

 which may develop and hatch rather quickly, hut can also lie 

 quiescent for a long period. These eggs are remarkably resistant to 

 adverse conditions; thev mav he dried and kept for several years 

 and then hatched when put into sea water. They may also he sub- 

 jected to more severe drying hy being kept in high vacuum over 

 phosphorus pentoxide, yet they still develop normally when put 

 into sea water. Cooling with liquid air is also survived, and a small 

 proportion of eggs are still viahle after being heated to 103 C. for 

 two hours. When these eggs are dry they seem to suspend all 

 activity, and the most refined techniques have failed to reveal any 

 trace of normal respiratory exchange. 



The cephalocaridan Hutchinsoniella carries two large eggs, each 

 in its own sac suspended from the front of the abdomen. 



Those copepods which do not cast their eggs freelv into the water 

 carry them in thin-walled sacs suspended from the front of the 

 abdomen. There mav be one sac. as in many calanoids and harpacti- 

 coids, or two sacs as in most cyclopoids. Some parasitic copepods, 

 such as the Choniostomatidae (p. 109) produce up to six or eight 

 egg: sacs. 



The young crustacean which emerges from the egg mav resemble 

 its parent rather closely, but more usually it is very different, and 

 has to go through a series of larval stages before attaining the adult 

 form. The nauplius is a very characteristic crustacean larva. Three 

 nauplii belonging to different groups are shown in fig. 33. The 

 main features which thev possess in common are a simple non- 

 segmented body and three pairs of appendages: the antennules, 

 antennae and mandibles. When the body is elongated and seg- 

 mented the nauplius becomes a metanauplius. The great interest 

 attached to the nauplius is due to its occurrence during the develop- 

 ment of so many different groups of the Crustacea. A nauplius or 

 metanauplius is found in at least one representative of each of the 

 following groups : Notostraca, Anostraca, Conchostraca. Cladocera 

 (from the winter egs of Leptodora onlvV Copepoda. Cirripedia. 

 Fuphausiacea. and the penaeid prawns. Tn addition the embryos 

 of many of the other groups go through a stage which is very 

 similar to a nauplius while still within the egg. 



The stage following the nauplius varies in the different groups. 

 There may be a gradual chanee through a number of moults 



