OF KESPIBATION. 



case, the filaments are traversed by blood-vessels, and are adapted to 

 bring the blood into close relation with the surrounding water ; and the 

 continual interchange of the latter is provided for by the restless move- 

 ments of the body. The tufts are sometimes arranged along every seg- 

 ment of the body ; and their multiplication prevents them from indivi- 

 dually attaining any considerable size. In other cases, they are disposed 

 'at intervals ; and they are then larger, being less numerous. Their most 

 beautiful development is where they are present on the head only, the 

 rest of the body being enclosed in a shelly or sandy tube, as in the Ser- 

 pulce and Terebellce. The gill-tufts then frequently present the appear- 

 ance of a flower, endowed, when alive, with the most brilliant and deli- 

 cate hues. In many animals of this group, there is a small supplemen- 

 tary heart at the base of every one of the vessels that distribute the 

 blood to the gills ; and this is obviously designed to aid in the respiratory 

 circulation, for which the feeble action of the dorsal vessel would not 

 furnish sufficient power ( 552). 



658. The higher Articulated classes are, for the most part, adapted 

 to atmospheric respiration, on the plan to be presently explained; but 

 there is one class, that of Crustacea, whose respiration is still carried 

 on through the medium of water. In the lowest forms of this group, 

 there is no special respiratory apparatus ; the general surface being soft 

 enough to admit of the required aeration of the fluids through its own 

 substance, and the animal functions being performed with so little 

 activity, that a very small amount of interchange is required. In the 

 higher orders, however, whose bodies are encased within a hard envelope, 

 we find external gills, like those of many Molluscs; and these are 

 attached to the most movable parts of the body, one or more pairs of 

 legs being in some instances kept in constant agitation, for the purpose 

 of producing currents in the surrounding fluid, that may serve for the 

 aeration of % the blood. In the Crab-tribe, which constitutes the highest 

 family of this class, the gills are themselves enclosed within a cavity, 

 formed by a sort of doubling of the hard integument of the under side 

 of the body ; and a constant stream of water is maintained through this, 

 by means of a peculiar valve, situated in the exit pipe ; the continual 

 movement of the valve causing a regular stream of water to issue from 

 the gill-chamber, and thus occasioning the entrance of a constantly-fresh 

 supply. In these, also, we find a dilatation, the walls of which seem to 

 have contractile powers, at the commencement of each artery that dis- 

 tributes the blood to the gills ; and this collects the venous blood from 

 the various channels, in which it has meandered through the body. It 

 is by the enclosure of the gills within a cavity, and by the consequent 

 protection of them from the drying influence of the air (which would 

 prevent their function from being duly performed), that Crabs and other 

 allied species are enabled to live for a considerable time out of water ; 

 and the Land-Crabs, as they are termed, are adapted to spend the greater 

 part of their lives at a distance from the sea, by means of a special 

 glandular apparatus within the gill-cavity, which secretes a fluid that 

 preserves the surface of the gills in the moist condition requisite for the 

 aeration of the blood through its membrane. Thus the Land- Crabs are 



