208 THE VITAL FUNCTIONS. 



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head, as being the only part which comes in contact 

 with the water. 



Most of the smaller Crustacea have branchiae in 

 the form of feathery tufts, attached to the paddles 

 near the tail, and kept in incessant vibratory mo- 

 tion, which gives an appearance of great liveliness 

 to the animal, and is more especially striking in 

 the microscopic species. The variety of shapes 

 which these organs assume in different tribes is 

 too great to allow of any specific description of 

 them in this place: but amidst these varieties it is 

 sufficiently apparent that their construction has 

 been in all cases designed to obtain a considerable 

 extent of surface over which the minute subdivi- 

 sions of the blood-vessels might be spread, in 

 order to expose them fully to the action of aerated 

 water. 



The Mollusca, also, present great diversity in the 

 forms of their respiratory organs, although they are 

 all, with but a few exceptions, adapted to aquatic 

 respiration. In many of the tribes which have no 

 shell, as the Thetis, the Doris, and the Tritonia, 

 there are arborescent gills projecting from diffe- 

 rent parts of the body, and floating in the water. 

 In the Lepas, or barnacle, a curious family, con- 

 stituting a connecting link between molluscous 

 and articulated animals, these organs are attached 

 to the bases of the cirrhi, or jointed tentacula, 

 which are kept in constant motion, in order to 

 obtain the full action of the water on the blood- 

 vessels they contain. 



We are next to consider the extensive series of 

 aquatic animals in which respiration is carried on 

 by organs situated in the interior of the body. The 



