218 OF RESPIRATION. 



its full development, among the air-breathing vertebrata, in- 

 stead of entering the general visceral cavity by tubes, and 

 flowing over the surface of the peritoneum by the motions 

 of cilia, as in the Asteriadae and Echinidce ; the water is 

 inspired through a single chamber, called the cloaca (g, fig. 

 232) ; and by the contraction of its muscular walls flows 

 into two tubular branched organs (i, k\ attached by a process of 

 the peritoneum to the walls of the body ; upon the membra- 

 nous lining of these organs, which divide and subdivide, like 

 a tree, into branches, terminating in tuft-like cells (in) ; the 

 blood-vessels ramify like the pulmonary vessels on the bron- 

 chial tubes in the air-breathing vertebrata, which they further 

 resemble in the rythmic movements of dilatation and contrac- 

 tion, which take place three times in a minute in the Holo- 

 thuria tubulosa (fig. 232), the water, after each inspiration, re- 

 maining about twenty seconds in the body. 



[ 379. The respiratory organs, in all the other classes of 

 the animal series, may be grouped into three principal forms ; 

 branchiae, tracheae, lungs. The plan manifested in the structure 

 of these organs is to fold up, into the smallest possible space, 

 a large extent of membranous surface, upon which a net-work 

 of blood-vessels may be spread. It is impossible to imagine a 

 more perfect fulfilment of these conditions than is accomplished 

 in the structure of the branchiae and lungs, whereby the whole 

 circulating fluid of the body is made to traverse a vascular net- 

 work, and is brought thereby into mediate or immediate con- 

 tact with the air of the atmosphere, or that held in solution in 

 the water : as a general rule, it may be stated that branchiae 

 are adapted for aquatic, and lungs for aerial respiration. 



[ 380. Most of the mollusca respire by branchiae. In the 

 TFNICATA they occupy the interior of a cavity which is tra- 

 versed by currents of water, entering at one orifice and escaping 

 at another, and caused by the vibrations of cilia. In the 

 Sa1p<B the branchia has the form of a tube, formed by a fold 

 of the internal membrane, disposed transversely in spiral turns, 

 which gives an annulated appearance to the cavity, and has 

 caused it to be likened to the tracheae of insects. The su- 

 perior border of this membrane is provided with an infinity of 

 small vessels, running parallel with each other ; in other genera 

 the branchia forms a more continuous lining of the respiratory 



