2i8 COMPARATIVE ANATOMY AND PHYSIOLOGY. 



oxygenated sea-water by a thin wall of membrane 

 through which oxygen passes inwards, and carbonic 

 acid outwards. In the Opliiuroidea, where the 

 calcareous skeleton consists of continuous plates, a 

 slit is to be seen on either side of the insertion of each 

 arm into the disc ; these so-called genital slits or 

 biirsal clefts, as they are more appropriately 

 named by Ludwig, lead into membranous sacs (bursse) 

 into which sea-water is sucked, and from which it 

 is again driven out ; the thin walls of these sacs 

 project into the body cavity, and here again the 

 oxygenated sea-water is separated from the contents 

 of the coalom by nothing more than a thin wall. In. 

 the Ecliinoidea, where the test is, again, con- 

 tinuous, the membranous sacs are either internal as 

 in Cidaris, where they lie at the apex of the lantern 

 of Aristotle, or they form five pairs of sacculated 

 projections which protrude from the five pairs of 

 slits in the margin of the mouth. In some Echinoids 

 the tube feet, or ambulacral suckers, take on a respi- 

 ratory function. In the Holotlmroidea, the respi- 

 ratory organs, when specially developed, are connected 

 with the intestinal tract. (See page 229.) 



A simple condition of external gill obtains in 

 some of the Polychretous Annelids, where, 

 however, they are by no means always developed. In 

 Nereis, for example, they are simple, short filaments ; 

 in Cirratulus the filaments are simple, but are produced 

 to an extraordinary length ; in others, as the lug-worm 

 (Arenicola), they are comparatively short but are 

 elaborately branched (Fig. 94) ; in the tube-dwellers 

 they are confined to the anterior end, where they form 

 a pair of " respiratory plumes." In all cases these 

 gills consist essentially of a thin wall, the epithelium 

 of which is ciliated, and within which are blood- 

 vessels more or less well developed. In the plumes of 

 the TiiMcolse supporting cartilages are not rarely 



