AQUATIC RESPIRATION. 297 



vascular net-work, containing the nutritive juices 

 of the animal, and apparently performing a 

 partial circulation of those juices. A still more 

 complicated system of respiratory channels 

 occurs, both in the Echinus 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 contact with all the organs. 



The animals composing the family of Ascidice 

 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 respiratory cavity, before it can 

 be swallowed, and received into the stomach. 



In several of the Annelida, also, we find in- 

 ternal organs of respiration. The Lumhricus ter- 

 restjis, or common earth-worm, has a single row 

 of apertures, about 120 in number, placed along 

 the back, and opening between the segments of 

 the body : they each lead into a respiratory 



