DEVELOPMENT OF THE CRAYFISH. 243 



worthy fact is that the development is completed within the 

 egg-case, and that it is continuous without metamorphosis. 

 In contrast to this shortening of the life-history, the young 

 lobster is liberated at what is called the Mysis or prawn 

 stage, the young Crab as a yet simpler Zoaa, the young 

 PencBUs as a Nauplius with only three pairs of appendages. 

 The shortened life-history of the crayfish is interesting in 

 relation to its fresh-water habitat, where the risks of being 

 swept away by currents are obviously great ; but it must also 

 be remembered that the tendency to abbreviate develop- 

 ment is a general one. It seems that there is some 

 maternal care in the crayfish, for the young are said some- 

 times to return to the mother after a short exploration on 

 their own account. 



Systematic Survey of the Class Crustacea. 



First Sub-Class. Entomostraca. 



These are the more primitive Crustaceans, often small 

 and simple, with a variable number of segments and append- 

 ages. Out of the egg a Nauplius is hatched. The adult 

 usually retains an unpaired frontal eye, and has no gastric 

 mill. 



Order i. Phyllopoda. 



Order 2. Ostracoda. 



Order 3. Copepoda. 



Order 4. Cirripedia. 



Order I. Phyllopoda. In these at least four pairs of swimming feet 



bear respiratory plates. The body is generally well segmented, 



and is protected by a shield-like or bivalve shell. The mandibles 



are without palps, and the maxillae are rudimentary. 



(a) Branchiopoda. The body has numerous segments and (10-20 



or more) appendages with respiratory plates. The shell is 



rarely wanting, usually shield-like or bivalved. The heart 



is a long dorsal vessel with numerous openings. The eggs 



are able to survive prolonged desiccation. 



Branchipus, a beautifully coloured fresh-water form, 



with hardly any shell. 

 Artemia. Brine-shrimps. Periodically parthenogenetic. 

 By gradually changing the salinity of the water, 

 Schmankewitsch was able, in the course of several 

 generations, to modify A. salina into A. miihlen- 

 hcf,usii, and vice versa. 



