SYSTEMATIC SURVEY OF CRUSTACEA. 



269 



into themselves in a way which early suggests their future digestive 

 function. On the surface of the egg there have already appeared 

 ectodermic thickenings the so-called eye folds, rudiments of the 

 appendages, and of the thoracic and abdominal regions. 



In the later stages invaginations of the ectoderm form the fore and 

 hind gut, which grow inward from opposite ends to meet the endoder- 

 mic mid gut, also the ear sac and the green glands. The gills are 

 formed in great part from ectodermic outgrowths or evaginations. 

 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 from 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 crayfish 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 wilhin 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 remembered that 

 the tendency to abbreviate development is a general one.N 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. 



(i) Entomostraca, 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. 



(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. 



{A pus, Bronchi- 

 pus, and Artemia 

 (brine-shrimps), 

 Daph nia^Moina, 

 Polyphemus. 



2. Ostracoda, Cypris, Cypridina. 



3. Copepoda, Cyclops, Arguhts, 



many parasites. 



4. Cirripedia, acorn shells and bar- 



nacles, e.g., Balamts and 

 Lepas. 



Leptostraca, e.g. 

 Arthrostraca, 



Thoracostraca, 



Nebalia. 

 Amphipods 



(sand hoppers, 



&c.). 

 Isopods (wood 



lice, &c.). 

 Cuma. 

 Squilla. 

 Mysis. 

 Shrimp, lobster, 



crayfish, crab. 



