264 



CRUSTACEA. 



mic mid-gut. The ear-sac and green gland, and the greater part of the 

 gills, have also an ectodermic origin. From the mid-gut the digestive 

 gland is budded out. The heart, the blood vessels, blood, and muscles 

 are due to the mesoderm. 



As usual, the nervous system arises fiom an ectodermic thickening. 

 The eye arises partly from the optic ganglia of the "brain," partly 

 from the "eye-folds," and partly from the epidermis. 



When the young crayfishes are hatched from the egg-shells, they still 

 cling to these, and thereby to the swimmerets of the mother. In most 

 respects they are miniature adults, but the cephalothorax is convex and 

 relatively large, the rostrum is bent down between the eyes, the tips of 

 the claws are incurved and serve for firm attachment, and there are other 

 slight differences. The noteworthy fact is that the development is com- 

 pleted within the egg-case, and that it is continuous without metamor- 

 phosis. 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 remem- 

 bered that the tendency to abbreviate development is a general one. 

 There is some maternal care in the crayfish, for the young are said 

 sometimes to return to the mother after a short exploration on their 

 own account. 



Systematic Survey of the Class Crustacea. 



(1) Entomostiaca, lower forms. 

 They are usually small and simple. 



The number of segments and ap- 

 pendages is very variable. 



The larva is generally hatched as a 



simple unsegmented Nauplius. 

 There is no gastric mill. 



(Apus, Branchi- 

 I pus,andArtemia 



1. Phyllopoda,-! (brine-shrimps), 



Daphnia,Moina, 

 \ Polyphemus. 



2. Ostracoda, Cypris, Cypridiiia. 



3. Copepoda, Cyclops, Argulus, 



many parasites. 



4. Cirripedia, acorn - shells and 



barnacles, e.g. Balanus and 

 Lepas. 



(2) Malacostraca, higher forms. 

 They are usually larger and more 



complex. 

 The head consists of 5, the thorax 



of 8, the abdomen of 6 (7 in 



Leptostraca) segments. 

 The larva is usually higher than a 



Nauplius. 

 There is a gastric mill. 



Leptostraca, e.g. Nebalia. 



{Amphipods (sand- 

 hoppers, etc.) 

 Isopods (wood- 

 lice, etc.). 



{Cuma. 

 Sqitilla. 

 Mysis. 

 Shrimp, lobster, 

 crayfish, crab. 



First Sub-Class. Entomostraca. 



Older 1. Phyllopoda. — In these at least four pairs of swimming feet 

 bear respiratory plates. The body is generally well segmented, 



