220 



OF RESPIRATION. 



[In the PTEROPODA, as the Clio and Hyalea, the branchiae 

 resemble membranous expansions, like fins, or lamellae, on 

 the surface of the body. In the CEPHALOPODA they form two 

 or four organs, lodged in a distinct chamber, into which the 

 water is inspired, and expelled through a funnel-like tube, 

 situated on the under side of the neck. 



[381. The CRUSTACEA present various phases of 

 branchial development ; in the lowest forms, no 

 special organ exists; the tegumentary membrane 

 forming a general aerating surface. In the bran- 

 chiopods, the last joints of the feet are flattened 

 and covered with a vascular membrane, adapted 

 for respiration ; these organs having a continual 

 oscillating movement. In the Squill a, the bran- 

 chiae are limited to the abdominal members ; 

 whilst in the decapoda, as the crab and lobster 

 (fig. 222), they are formed like those of mollusca 

 and fishes, and lodged in separate cavities under 

 the thoracic shield ; the renewal of the water being 

 effected by the motion of distinct appendages. 

 In those Crustacea, as the land crabs, which live 

 for a time on shore, the branchiae are kept moist 

 by the membrane lining, the cavities being disposed 

 in folds, to serve as reservoirs for water ; and 

 sometimes it presents a spongy texture for the 

 same end. 



[ 382. The marine AKNELIDA respire by bran- 

 chiae variously disposed, on different parts of their 

 bodies ; in those living in tubes, as Serpula and 

 Sabella, they resemble the tentacula of polyps, and 

 form plumelike coloured organs, sometimes with a 

 spiral winding. When fully expanded in the water, 

 they are adorned with the most beautiful colours. 

 In the Amphitrite they are pectinated ; in Terebelhe 

 they resemble small trees planted round the neck. 

 In the genera which swim freely through the 

 water, they are disposed in longitudinal lines ; in 

 the Arenicola (fig. 233), they form a series of tufts, 

 rich in bloodvessels. In Eunices, they have a 

 of pectinated form, and in Aphrodita they are placed 

 the Arenicola. on scales along the back. In the Ilirudo (fig. 





