COM PA KATI V K ANAT< >M V. 



2S1 



tofts of vesseU derived exclusively from the closed YMonlar 



but in tlioae oases before alluded to, where the gilla are 



more liko solid platen, not only ia the fluid of the vessels eon- 



into them ; but they are ohannolled into spaces into which 



the ohyluquooua fluid of the abdominal cavity can tiiul its way, 



and thus becomes exposed to the influence of the oxygon of the 



When the gills are almost wholly composed of protrusions of 



than most worms, being of an oral form. That which u most 

 attractive about thin otherwise inert and uninteresting animal 

 is the splendid play of colour* which glance* from the thickly- 

 set bristles which clothe its aides. The bristle* are from their 

 shape not only locomotive organs, bat > of defence; foi 

 many of them are found, under the microscope, to be "%11 

 harpoons, furnished with many barbs. These, like the oar- 

 bristles of other families, are capable of being withdrawn into 



I. NEREIS (A SEA-WORM). II. TRANSVERSE SECTION OF THE EUNICE. III. PROBOSCIS OF A NEARLY ALLIED SPECIES. IV. LEECH, SHOWIHO 



THE BODY-WALL AND ALIMENTARY CANAL LAID OPEN ALONG THE BACK. V. DIAGRAM SHOWING THE CIRCULATION IN THE LEECH. VL 



ORAL SUCKER OF A LEECH. VII. LONGITUDINAL SECTION OF AN EARTH-WORM, SHOWING THE CIRCULATION. Till. DIAGRAM SHOWING 



THE CIRCULATION IN A TRANSVERSE SECTION OF A WORM AT THE TAIL END OF THE ANIMAL. IX. DIAGRAM SHOWING THE TRAN8VKRSE 

 CIRCULATION IN A IiOB-WORH (ARENICOLA). 



Befs. to Nos. in Fig. II. 1, comb-like grill ; 2, dorsal feelers ; 3, bundles of bristles ; 4, ventral feelers ; 5, cavity of alimentary canal ; 6, dorso- 

 intestinal vessels ; 7, ventral vessel ; 8, vascular loop acting as a heart to force the blood to the gills ; 9, plexus of vessels 

 the gills. 



the vascular system through the body wall, since the blood in 

 them is kept in constant and rapid circulation, it is necessary 

 that the outer sea-water should bo as rapidly changed ; and 

 hence we find such gills are clothed with those minute, con- 

 stantly-moving hairs which we call cilia. On the other hand, 

 when the gills are in the shape of massive lobes so channelled 

 out as to admit of the chylaqueous fluid to remain in them, 

 and to bo only changed slowly by the motion of the body, the 

 necessity of change of the external water is not so pressing, and 

 therefore these lobes are naked and not ciliated. 



The representative of one of the families of sea-worms is the 

 common sea-mouse (Aphrodite). It is much broader and shorter t 



pits made by the inversion of the papillae on which they are 

 set. Lest the harpoons should wound the skin when with- 

 drawn, each is furnished with a sheath consisting of two pieces, 

 which are made into a split tube holding the retracted weapon. 

 The common sea-mouse has two stalked eyes and three tentacles 

 on its head. One great peculiarity of its structure is that its 

 back is covered in with a coating of felt composed of tangled 

 and matted hairs. This felt covering is not continuous, bat 

 consists of pairs of plates attached to certain segments of the 

 body, the hind edges of the front plates overlapping the front 

 edges of those which come behind. These plates are moved 

 up and down by muscles, which are capable of erecting and 



