32 RESPIRATORY MECHANISMS 



Among the Cirripedia, filamentary appendages attached to 

 the cirri in 4 genera of Lepadidce are described as gills, but their 

 function as such is very doubtful, and the large majority of 

 the species do well without (Darwin, I, p. 38, 1851). In the 

 Balanidce special "branchial" organs are attached on each side 

 to the wall of the "sack," and ventilating movements are de- 

 scribed by Darwin (II, p. 63, 1854). No circulatory organs 

 have been detected, and it is relevant to point out that without 

 circulation the "gills" cannot possibly provide oxygen for more 

 than their own substance. 



Small and very primitive gills are found in the Asteridce 

 while those in the Echinoida are generally more elaborate and 

 specialized. The water outside and the coelomic fluid inside 

 these organs is kept in motion by cilia. In both groups a 

 considerable proportion of the exchange no doubt takes place 

 through the tube feet and other appendages. 



In many Polychceta {Nereis) the parapodia are richly sup- 

 plied with blood and function as gills, while in others (Areni- 

 cola) special appendages, with feathering branches growing out 

 from the parapodia, have taken over most of the function. 

 In some Polychceta (Dasybranchus, Eisig, 1887) the branchial 

 tufts can be readily retracted into the body, and such move- 

 ments are regularly utilized to renew the water along their 

 surface. Such renewal takes place in many cases by ciliary 

 movements (nudibranchs), in others by the locomotor ac- 

 tivity of the organism as a whole (Phyiiopoda), while in some 

 animals muscles cause irregular or rhythmic movements of 

 the branchial tufts (Amphibia, Ephemerid larvae), and in others 

 again there are special appendages which by their movements 

 produce a flow of water along the gills (many Crustacea). 



In a fairly large number of insect larvae (Ephemeridce, 

 Odonata, Trichoptera, Coleoptera) we find free tracheal gills. 

 So far as the exchange of gases is concerned these function 

 just like blood gills, but the transport mechanism between the 

 gill and the rest of the body is not a circulating fluid, but diffu- 

 sion in air tubes which can be effective because the rate of 



