232 



BIOLOGY. 



[BOOK ir. 



plays also a respiratory part, a part the more important the more 

 rapid the renewal and the circulation of the water are. 



The branchiae appear in the annelates. They are, as in most 

 of the invertebrates, appendices projecting from the external 

 tegument. Different organs, particularly the dorsal cirrhi, grow 

 vascular and become branchial in tufts more or less ramified 

 (Fig. 25). 



FIG. 26. 



Intestinal canal and ramified organs of a Iwlothurion: o, mouth ; t, intestinal tube; d, 

 cloaca ; a, amis ; c, ramified stony canal ; p, Poll's vesicle ; rr, arborescent organs ; /, 

 their union at the point of intersection above the cloaca ; m, longitudinal muscles of the 



In the tunicians there is a sort of respiratory pouch ; it is the 

 anterior part of the digestive tube. This portion dilates into a 

 sac, at the bottom of which the true digestive tube commences. 



In the holothuria exists a system of aquiferous canals, 

 probably respiratory ; it is a double tree, the two orifices whereof 

 open into the cloaca. This cloaca receives the water, and projects 

 it vigorously outwards, on an average three times a minute 



