V 



AQUATIC RESPIRATION. 269 



first example of internal aquatic respiration occurs 

 in the Holothuria, where there is an organ com- 

 posed of ramified tubes, the stem of which commu- 

 nicates with the expanded end of the intestine. 

 This organ is situated in a cavity having an ex- 

 ternal opening for the admission of the aerated 

 water, which is brought to act on a vascular net- 

 work spread over the tubes, containing the nutri- 

 tive juices of the animal, and apparently perform- 

 ing a partial circulation of these juices. A still 

 more complicated system of respiratory channels 

 occurs, both in the Echimis and Asterias, 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 con- 

 tact with all the organs. 



The animals composing the family of AscidicB 

 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 en- 

 trance into the oesophagus, or true mouth, is si- 

 tuated at the bottom of this cavity ; that is, at the 

 part most remote from the external orifice ; so that 

 all the food has to pass through the respiratory 

 cavity, before it can be swallowed, and received 

 into the stomach. 



In several of the AnneUda, also, we find internal 

 organs of respiration. The Liimhricus terrestris, or 

 common earth-worm, has a single row of apertures, 

 about 120 in number, placed along the back, and 



