126 



INSPIRATORY SYSTEM. 



(PL X, figs. 63, 64; PL XI, figs. 65, 66, 67). 



Respiratory Mechanism. 



Respiration is effected by means of gills or branchiae, 

 which are outgrowths from the Avails of the thorax. The 

 gills do not project directly on to the exterior, but are 

 situated in the branchial chambers at each side of the 

 cephalothorax. 



The branchial chamber s — one pair. The 

 cavity of each chamber is morphologically a part of the 

 exterior, and its walls are formed by the downgrowth of 

 the carapace at each side. The sub-branchial region of 

 the carapace is closely applied to the coxopodites of the 

 pereiopods, and here turns inward to form the wall of the 

 branchial chamber. In transverse section the chamber 

 has a triangular shape (PL IX, fig. 56), and its walls may 

 be spoken of as ventral, dorso-lateral and inner. The 

 two former are membranous and are continuations of the 

 inturned edge of the sub-branchial region. 



The postero-lateral portion of the digestive gland 

 rests upon the roof of the branchial chamber. Between 

 the floor of the chamber and the sub-branchial region of 

 the carapace there is a mass of connective tissue. In the 

 anterior region of the chamber the floor is raised into a 

 well defined transverse ridge. In a full-sized crab this 

 groove is about ^ inch in front of the anterior inhalent 

 aperture. In sections through this ridge the epidermis is 

 greatly elongated and has a glandular structure. There 

 are also tegumentary glands below the epidermis. The 

 podobranch of the second thoracic somite is closely 

 applied to the posterior side of this ridge. For the sake 

 of convenience I designate the latter the branchial ridge. 

 Its probable function will be discussed below. 



