530 ORGANS OF RESPIRATION. 



trunk, but they are individually more ramified and length- 

 ened, and they exhibit most regular movements of expan- 

 sion and contraction as they receive or expel the red blood 

 transmitted by the branchial vessels. In the common areni- 

 cola piscatorum, there are thirteen pairs of these branched 

 organs along the dorsal part of the body, and three or four 

 segments of the trunk intervene between each pair of bran- 

 chiae. In polynoe, the branchiae are in form of thin dilatable 

 membranous sacs disposed externally along the whole extent 

 of the sides of the trunk. The branchiae are also in form of 

 membranous sacs placed along the dorsal part of the body in 

 aphrodita (halithea, Lam.) where they are sometimes con- 

 cealed by an external fibrous reticulate covering, as in halithea 

 aculeata, or exposed on the exterior of the back, as in hali- 

 thea squamata. There are thirty compressed dorsal respi- 

 ratory sacs in the halithea aculeata, disposed in fifteen sym- 

 metrical pairs, along the whole extent of the body, with 

 fourteen pairs of much smaller crested or serrated sacs 

 interposed between the larger. They all communicate, at 

 their inferior margin, with a distinct closed dorsal respiratory 

 cavity, extending along the whole body, and separated by its 

 thin parietes from the abdominal cavity, and from the re- 

 ticulate texture covering the back. The sacs open also 

 externally by minute compressed lateral stigmata concealed 

 in deep depressions between each pair of feet, and there are 

 t\vo large openings into the general respiratory cavity, placed 

 at the sides of the buccal orifice. 



There are nearly twenty pairs of isolated internal, highly 

 vascular, respiratory sacs, disposed along the whole abdomi- 

 nal cavity of the leech, hirudo, lined each with a soft se- 

 reting mucous membrane, covered with a peritoneal coat, 

 supplied with vessels from the great lateral trunks, opening 

 by small round stigmata on the ventral surface of the body, 

 and the pairs separated from each other by about five seg- 

 ments of the trunk. Similar minute, isolated, internal, late- 

 ral, respiratory sacs are disposed in regular pairs along the 

 whole extent of the abdominal cavity in the earth-worm, 

 lumbricus, which, like those of halithea, and hirudo, are 

 largest in the middle portion of the trunk ; they become 

 almost imperceptible towards the ends ; they generally con- 

 tain a white mucous fluid, and they open separately by 



