BLOOD SYSTEM. 



399 



Arthropods: Crustacea.— As an example of the 

 department of arthropods, therefore, we select crusta- 

 ceans. 



Respiratory Organs. — Take a crab, lobster, or crawfish, 

 and remove the carapace or dorsal shell. Directly 

 exposed on each side and occupying a large part of the 

 upper surface are seen 

 a great number of taper- 

 ing, finely lamellated or 

 tufted organs, Fig. 276, 

 G G. These are the 

 gills. They are not with- 

 in the body cavity, but 

 wholly outside, in special 

 respiratory chambers, 

 opening by a large cleft 

 on each side of the shell 

 (Fig. 277). The gills 

 are, some of them, con- 

 nected each with a limb 

 or a maxilliped, and are 

 indeed appendages of 

 these, and some with the 

 thoracic walls. The fine 

 mosslike structure is a 

 device for producing as 

 large a surface as pos- 

 sible of exposure of the 

 blood to the aerated 

 water. 



Breathing. — The exchange of water is effected partly 

 involuntarily by ciliary currents, partly in some by the 

 action of the maxillipeds, and partly — i. e., when in active 

 locomotion — by the movements of the limbs, and there- 

 fore of their appendages, the gills, 

 27 



Fig. 277. — Transverse section through a 

 crawfish, showing the gills : gch, gill 

 chamber ; H, heart ; /, liver ; z', intes- 

 tine ; vs, blood sinus. The arrows 

 show the course of the blood. 



