232 PHYSIOLOGY OF THE INVERTEBRATA. 



(brancliia3) are couverted into air-breathing organs in the 

 land crabs, being protected and kept moist in a large 

 chamber formed by the carapace." These branchiae are 

 supplied with blood which is returning to the heart. 



The gills or branchia? are composed of either broad lamellas, 

 or these lamellae are divided into filaments, giving the gill its 

 plume-like appearance. As already stated, it is essential that 

 the water in contact with the branchi^ should be constantly 

 changed. This change is effected by various devices. Thus, 

 in Astacus, Homarus, and the higher Crustacea the branchias 

 are placed in a chamber, which is bounded externally by the 

 branchiostegite, and internally by the lateral walls of the 

 thoracic segments. It is open below and behind, between 

 the bases of the thoracic limbs and the free edge of the 

 branchiostegite. Behind the anterior opening of the chamber 

 (on each side of the body) lies the scaphognathite (a broad 

 fringed organ), which moves continually backwards and 

 forwards, baling out the impure water of the chamber (i.e., 

 the water impregnated with COg), and thus compelling fresh 

 or oxygenated water to flow in through the posterior aperture 

 and over the branchia). 



The number of branchias varies considerably in different 

 Crustaceans (e.g., in Astacus there are eighteen in each 

 chamber, and in Nephrons there are twenty). 



The respiratory function in all Branchiopoda is performed 

 by the branchial feet; but according to Dr. G. 0. Sars,* 

 there is, however, another part of the body in the Phyllopoda 

 (i.e., in Cyclestheria hislojn) that apparently can lay claim 

 to a true respiratory function. Sars, as well as others, state 

 that the valves of the shell, which receive a consider- 

 able quantity of blood, and their, inner delicate coating, 

 seem highly calculated to produce an exchange of gases 

 with the water. The necessary renewal of the water is 

 effected by the well-nigh uninterrupted movements of the 

 legs, whereby a continual current is produced within the 

 * Christiania Viikmlcahs-i^elskahs Forhandlingen, 1887. 



