296 THE VITAL FUNCTIONS. 



amidst these varieties it is sufficiently apparent 

 that their construction has been, in all cases, de- 

 signed to obtain a considerable extent of surface 

 over which the minute subdivisions 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 different parts of the body, and floating in 

 the water. In the Lepas, or barnacle, a curious 

 family, constituting 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 first example of internal aquatic respiration 

 occurs in the Holothuria, where there is an 

 organ composed of ramified tubes, situated in a 

 cavity communicating with the intestine, and 

 having an external opening for the admission of 

 the aerated water, which is brought to act on a 



