224 PHYSIOLOGY OF THE INVERTEBRATA. 



which are usually called respiratory ; and certainly this form 

 of respiration is well developed in Hirudo. 



In the Polijchccta there are simple or branched cirri or 

 branchia3 which have ciliated walls and contain blood-vessels. 

 These branchiae, situated on the dorsal walls, are usually 

 appendages of the parapodia, and have a respiratory function. 

 In the lower Invertebrates the branchi^ are mostly situated 

 externally {e.g., Arcnicola), so as to float freely in the sur- 

 rounding water ; whilst in the more highly organised, as the 

 Molluscs, these organs are enclosed in a cavity into which 

 the water has free access, and may easily be renewed. 



In the Gepli]irca there appears to be several devices all 

 aiding in the function of respiration. 



(«.) A respiratory function is attributed to the tentacula 

 in these marine vermiform animals. 



(b) In Priapuhis and Sternaspis there are certain fila- 

 mentous appendages given off" at the posterior end of the 

 body. These are said to be branchia3. 



(c) The pseudo-htemal system is present in most of the 

 Gephyrca, and it has a respiratory function. 



(d) "In Echiurus, Bonellia, Thalassema, a pair of tubular, 

 sometimes branched organs, which are ciliated internally, 

 and communicate by ciliated apertures with the peri- 

 visceral cavity, open into the rectum. These appear to 

 represent the water-vessels of the liotifcra and the respiratory 

 tubes of the Holothuria." 



Although the above devices represent the actual organs 

 of respiration in the Annelida, supplementary respiration, 

 by means of pigments, also occurs in this class of animals. 



In Phyllodocc viridis there occurs a green pigment which 

 is not chlorophyll.* This pigment is soluble in alcohol and 

 benzene. MacMunn examined a living specimen of this 

 Polycha3te Annelid in the compressorium, under the micro- 

 spectroscope, but he could not detect any bands ; in fact, he 



* Prof. P. Geddes in Proc. Roy. Soc. Ed'in., vol. ii, p. 379; and Dr, 

 MacMunn in Quart. Journ. Micros. Science, vol. 30, p. 70. 



