RESPIRATION IN SHORE ANIMALS 203 



protected, a circulation of the water over the respiratory 

 tissue is only a degree less necessary and is obtained in 

 various ways. The method in fishes hardly needs descrip- 

 tion. Water is taken in at the mouth and expelled through 

 the gill-slits, or the water may enter underneath the gill- 

 cover and leave by the same opening. In crabs and lobsters, 

 where the gills are very completely protected, a current of 

 water is kept passing through the gill-chambers from behind 

 forwards by the regular action of a specially modified appen- 

 dage — the baler or scaphognathite. 



Respiration in sea-cucumbers shows a very curious 

 convergent resemblance to the same process in a land 

 vertebrate. The respiratory organs in this group are 

 mainly the " respiratory trees " (known in French as 

 " poumons "), hollow, branched structures situated within 

 the body cavity of the animal and communicating indirectly 

 with the exterior by means of the cloaca. As a result of the 

 rhythmical contraction of the cloaca, the lower portions of 

 the trees are alternately filled and emptied, an exchange of 

 gases taking place between the water, when the trees are full, 

 and the blood contained in their walls. 



In the so-called " current-feeders " the water which is 

 caused to flow through the body by means of ciliary action 

 serves to convey the food and, at the same time also, the 

 oxygen necessary for respiration. The tentacular crown — 

 when this is present— and parapodial cirri of bristle- worms, 

 notwithstanding their exposed character and the fact that 

 the water surrounding them is in constant motion, are also 

 ciliated, as are analogous outgrowths in other groups, e.g. 

 Nudibranch molluscs. 



To the student who is thinking over the process of 

 respiration in shore animals a word of caution is necessary. 

 The tendency is to regard any delicate vascular outgrowths 

 as respiratory. This is a common-sense view, but it is 

 doubtful, however, whether it is in every case borne out by 

 facts. The removal of the so-called gills of certain tube- 

 worms, for instance, has been shown to have little adverse 

 effects on these animals from the point of view of breathing. 



