33^ 



PHYLUM ARTHROPODA 



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

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

 dermic mid-gut. The ear-sac 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 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 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 like the 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 completed within the egg-case, and that it is continuous without 



Fig. i8o. — Section through cephalothorax of a crab. — 



After Pearson, 



H., Heart ; Te., extension of the terguin ; ST., sternum ; PL., pleuron ; 

 T., tendons ; ist W.L., insertion of first walking leg ; Br., gill in gill- 

 chamber ; g., gut; d.a., descending artery; A., afferent branchial; 

 E., efferent branchial. 



metamorphosis. 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. 



The Crab 



It is instructive to contrast the crab-type with that of the crayfish 

 or lobster. The cephalothorax is broadened by a hollow extension of 

 the gill-covering (branchiostegite) region. The abdomen is greatly 

 reduced, with a soft sternal region, and is bent permanently upwards 

 and forwards in a groove in the thoracic sterna. In the male there are 

 only two pairs of abdominal limbs, which have a reproductive function ; 

 in the female there are four pairs, which carry the eggs. 



