454 



THE POPULAR SCIENCE MONTHLY. 



cucumber, where it is connected with gill-like appendages about the 

 mouth. The intestinal worms and other annelids possess this style of , 



breathing-organs. Later we shall find in 

 the insects a similar and perhaps homolo- 

 gous system for conveying air. 



Most aquatic invertebrate animals 

 which have a true circulating fluid send 

 this to expansions of the skin to be there 

 oxygenated by the surrounding water. 

 This is termed. " branchial " respiration. 

 The branchiae or gills may be wholly ex- 

 posed and external to the body, or may 

 be contained in suitable cavities. They 

 may develop on any part of the body ; 

 and they exhibit a great variety in many 

 respects. With this system is frequently 

 found some special accessory apparatus 

 for producing and regulating a flow of 

 water over the gills. 



The sea-worms have a great variety 

 of external gills developed immediately 

 from the skin. Renewal of water is se- 

 cured by cilia which cover the body or 

 even the gills. These branchiae are finely 

 shown in the common Serpula, where 

 they form a crown of scarlet plume-like 

 about the mouth. The Arenicola, or 

 sand- worm, has two rows of crimson 

 rosettes along the sides of the body. 

 The Eunice, Nereis, and sea- mouse, are 

 other worms possessing beautiful arbo- 

 rescent fringes of branchiae. 



As the crustaceans, excepting a few of 

 the arrows indicating the" direction the lowest, are covered with a hard shell of 



or the currents ; a ', adductor nine- ' 



cies ; 6,;riiis; a heart ; o mouth, sur- dead matter, they can not respire through 



rounded by (p) labial palpi ; /, toot; . . r 



v, anus ; m, cut edge of the mantle, the skin, and hence of necessity require 



(After Woodward.) J \ 



special organs. 1 hese are commonly leaf- 

 like expansions, usually attached to the locomotive or other append- 

 ages ; the purpose of such attachment being to secure rapid change 

 of the surrounding water. In some groups, the entire limb becomes 

 foliaceous and respiratory, so that it is literally correct to say that the 

 fairy shrimp breathes with its legs. Other forms, including the sand- 

 hopper, whale-louse, and fresh-water shrimp, breathe by vesicles or 

 bladders attached to the limbs. The water-fleas, as Cypris and Cyclops, 

 so common in our fresh waters, have their branchiae attached to the 

 jaws. Many forms, among which are the king-crab, or Limulus, and 



Fig. 7. Anatomt or a Bivalve Mol- 

 lusk (Mya arenti7-ia). The left, valve 

 and mantle-lobe and half the siphons 

 are removed, ss, respiratory siphons, 



