528 ORGANS OF RESPIRATION, 



THIRD SECTION. 



Respiratory Organs of the Diploneurose or Articulated 

 Classes. 



In the sluggish helminthoid classes of articulata the respi- 

 ration is nearly always aquatic, but in the active entomoid 

 classes it is most generally aerial; and in the simpler forms of 

 ivorms, as in the lower radiata, no special organ appears to be 

 appropriated to this function. From the internal habitation 

 of the entozoa, and the general softness and vascularity of 

 their exterior covering and their component tissues, they 

 appear to be oxygenated directly by the contact of the fluids 

 of the animal in which they are parasites, like a foetus in 

 utero, an ovum, a gemmule, or a portion of the body itself; 

 the possession of special pulmonary or branchial appa- 

 ratus, would be unavailing in animals permanently buried in 

 the substance of the brain, the liver, or similar internal 

 organs, and with so limited a circulation of the blood, and 

 where their temperature is not dependent on their own vital 

 functions. 



The subcutaneous situation of the transverse abdo- 

 minal blood vessels of the rotiferous animalcules, and of 

 their vascular plexus around the neck, their thin soft and 

 generally naked skin, and the ciliary currents almost con- 

 stantly flowing over its surface, together with the ciliary 

 movements produced in their wide buccal and gastric cavities 

 and even in their general peritoneal abdominal cavity, contri- 

 bute to the extensive aeration of their ever-active body, inde- 

 pendently of the distinct branchial organs which have been 

 detected in several species. The branchiae are here placed in 

 the interior of the abdominal cavity, as in holothuria ; they 

 are furnished with large vibratile valvular cilia, like many 

 Crustacea; they communicate with the exterior medium by 

 a single median dorsal aperture near the neck, and they are 

 symmetrically disposed on the two sides of the abdominal 

 cavity. Eight of these branchial organs, placed apart from 

 each other, and attached to internal lateral tubes, apparently 

 the testes, are seen in the transparent abdomen of the hyda- 

 tina sent a, with their vibratile laminae in constant action. 



The branchiae of the cirrhopods are short laminated 



