Respiration in Invertebrate Animals. 399 



and simultaneous retraction of all the branchiae into the interior 

 of the body, which follows when the animal is touched. This 

 sheathmg of the blood-vessels with true muscular coats is a fre- 

 quent character of the circulating system in the Annelida. In 

 Arenicola, as in all Annelids in which the vessels are naked, the 

 branchiae are destitute of vibratile cilia. 



To the frequenter of the sea-shore the preceding description 

 of the mechanism of breathing in the familiar lug cannot prove 

 unacceptable. Its faecal coils are encountei'ed at every step. 

 The animal is ceaselessly occupied in swallowing and rejecting 

 wet sand. A considerable amount of water and sand is incessantly 

 traversing the body of the animal from one extreme to the other. 

 The organic particles are appropriated during the digestive pro- 

 cess ; the water in part yields up its oxygen and in part replenishes 

 the large volume of chylaqueous fluid with which the visceral 

 cavity is distended, — another example of internal respiration. 



Provided the branchiae convey to the surrounding medium the 

 blood-proper exclusively, and these organs occur in form of naked 

 vessels projecting above the external surface, the description now 

 given of the branchiae of Arenicola will apply in every minute 

 respect of structure to all other Annelida. It will prove exact 

 in relation to the structure of the gills in the several species of 

 the beautiful genus Euphrosyne of Savigny. 



In Euphrosyne laureata they rise under the protection of the 

 setae as brightly florid brushes on the back. They are fixed by 

 means of three or four primary trunks. Viewed by transmitted 

 light and under a high power, each ramuscule is seen to consist 

 of a single leafed vessel embraced in a very attenuated musculo- 

 membranous sheath. They are destitute of ciliary epithelium. 



xln Annelid of great beauty of figure is described by Milne- 

 Edwards and Audouin under the name of Hippono'e Gaudic/iaudii, 

 in which the branchial appendages assume the character of 

 arbuscles of naked vessels garnishing most ornately the entire 

 dorsum of the animal. Pleione tetrahedra, the typical species of 

 the genus Amphinome, exhibits the breathing organs under the 

 figure of scarlet bunches mounting round dorsadly each annulus 

 of the body, and guarded in front by a bundle of strong bristles. 

 The branchiae in Pleione Alcyonia afi*ect a ventral situation, and 

 exhibit a much less ornamental character. 



Chloeia capillata (Savigny) is an Annelid of matchless beauty. 

 The whole line of the back on either side of the median line is- 

 decorated with arborescent vermilion tufts. Each tuft is sup- 

 ported by single contracted stems embracing two trunks. They 

 are clothed with a slender musculo-cuticular rmciliated mem- 

 brane. In this rare worm the chylaqueous fluid is abundant, 

 yet no external organs are furnished for subjecting it to the 

 agency of the aerating element. 



