212 THE VITAL FUNCTIONS. 



sions of the blood vessels might be spread, in order to ex- 

 pose 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 few 

 exceptions, adapted to aquatic respiration. In many of the 

 tribes which have no shell, as the Thelis, the Doris, and the 

 Tritoiiia, 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 ofgans 

 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 Holothicria, 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 vascular net-work, containing the nutri- 

 tive juices of the animal, and apparently performing a par- 

 tial circulation of those juices. A still more complicated 

 system of respiratory channels occurs, both in the Echinus 

 and Ssterias^ where they open by separate, but very minute 

 orifices, distinct from the larger apertures through which the 

 feet protrude; and the water admitted through these tubes is 

 allowed to permeate the general cavity of the body, and is 

 thus brought into contact with all the orojans. 



The animals eomiposing the family of Jiscidix have a large 

 respiratory cavity, receiving the water from without, and 

 having its sides lined with a membrane, which is thrown into 

 a great number of folds; thus considerably extending the 

 surface on which the water is designed to act. The entrance 

 into the oesophagus, or true mouth, is situated at the bottom 

 of this cavity; that is, at the part most remote from the ex- 

 ternal orifice; so that all the food has to pass through the 



