CRUSTACEANS. 



481 



As an illustration of this metamorphosis, we give figures of the 

 Zoea Taurus in two states, viz., Fig. a, in the first stage ; and second, 

 Fig. I, as the animal appeared on the fourth day after the first micro- 

 scopic examination, and when it resolved itself into a kind of prawn. 

 The drawings appear in Mr. Bell's " History of British Stalk-eyed 

 Crustacea," and were taken by that gentleman from the work of a 

 Dutch naturalist, named Slabber, who made the original observation 

 in the year 1768, and published the result in 1778, from which 

 time the subject had been allowed to fall asleep until revived by Mr. 

 Thomson. 



Among the sea-spiders, which have no neck (Cephalotlwrax), the 

 head gradually disappears in the breast, but the belly remains distinct ; 

 the middle of the body is compressed, the shape narrow and graceful. 

 Among the Crusta- 

 ceans which have 

 neither neck nor 

 shape, the head, the 

 breast, and the belly 

 form only one mass, 

 often short, squat, 

 athletic, and difficult 

 to take, as in Pisa 

 ietraodon (Fig. 332), 

 the four-horned spi- 

 der-crab. 



Many of these ani- 

 mals have a powerful 

 tail, consisting of a 

 certain number of 

 ciliated paddles, which it uses in swimming to beat the water, and 

 to confuse its enemies. 



The Crustaceans, so far as they are aquatic, respire by means of 

 Iranchise, or gills. In the larger species these branchiae are lamellous, 

 or with filaments, whose supports are traversed by two canals, one of 

 which leads the blood into the general economy, the other directs it 

 towards the heart. These organs are enclosed in the body. In the 

 smaller species the branchiae often appear exteriorly, hanging in the 

 water like a fungus. Sometimes these are at once swimming and 



2 i 



Fig. 332. Pisa tetraodon. 



