ORGANS OF RESPIRATION. 529 



pyramidal organs, varying in number, size, and form, 

 in different species, attached to the sides of the trunk, 

 or to the bases of the proximal pairs of articulated feet, like 

 those attached to the haunches of the ]egs in the higher forms 

 of Crustacea ; and they aerate the blood spread upon their 

 numerous component laminae, before it is transmitted to the 

 pulsating dorsal vessel for distribution through the system. 

 The red-blooded worms have the greatest activity, and the 

 most extensive sanguiferous system of all the helminthoid 

 classes, and they present a corresponding development of the 

 respiratory organs, which are most generally branchial, 

 sometimes external, sometimes internal ; some respire by 

 the naked ciliated surface of their body, and a few possess 

 internal pulmonary sacs for an aerial respiration. The 

 vascularity of the skin, and the incessant currents of water 

 over its exterior, produced by minute vibratile cilia which 

 cover its naked surface, appear sufficient to oxygenate the 

 fluids in some of the lower annelides, as the planarice, which 

 present no special organ appropriated to respiration. No 

 distinct branchiae have been seen in the nais, the gordius, 

 and some other genera of the abrunchia of Cuvier; but the 

 tubicolons annelides, whether secreting calcareous shells, or 

 constructing adventitious tubes of sand or mud, are generally 

 distinctly cephalo-branchiate, having regular ramified tufts, 

 or elegant plumose expanded branchiae, covered with vibratile 

 cilia, and symmetrically disposed on the head or anterior 

 part of the body, as in sabella, serpula, terebella, amphitrite, 

 and pectinaria. These organs, in the living animals, are 

 commonly a little extended from the orifice of the tubes, 

 and expanded to receive the full influence of the ciliary 

 currents, on the red blood distributed through their minutest 

 ramifications. Most of the naked and burrowing aquatic 

 annelides are dorsibranchiate, having the branchiae in elegant 

 arborescent tufts, symmetrically arranged along the exterior 

 and dorsal aspect of the segments, or in form of more simple 

 branchial sacs confined to their interior. In amp/iinome, 

 hipponoe, pleione, and nereis, these ramose branchial tufts 

 accompany the upper pairs of lateral feet nearly along the 

 whole extent of the trunk. They are more concentrated in 

 their distribution in arenicola, where they are confined to a 

 limited number of the segments in the middle portion of the 



TART VI. M M 



