AQUATIC RESPIRATION. 297 



act on a vascular net-work, containing the nutri- 

 tive juices of the animal, and apparently per- 

 forming a partial circulation of those juices. A 

 still more complicated system of respiratory 

 channels occurs, both in the Echinus and Aste- 

 rias, where they open by separate, but very 

 minute orifices, distinct from the larger aper- 

 tures 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 Liimhricus ter- 

 restris, 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 



