202 THE BIOLOGY OF THE SEA-SHORE 



in addition, scattered over their dorsal surface, delicate 

 finger-like extensions of the body wall which contain an 

 extension of the body cavity. Both inside and out these 

 processes are lined by cilia, the internal cilia serving to 

 keep in motion the coelomic fluid and the outer ones to 

 ensure the constant changing of the water (Delage and 

 Herouard, 1903). 



In every group there are animals of small size which 

 are able to obtain sufficient oxygen through their skin, over 

 their whole surface. No special organs of respiration occur 

 at all within the groups of sponges and coelenterates. 



A special respiratory pigment is of common, though 

 not universal occurrence, in the blood or coelomic fluid of 

 marine invertebrates : haemoglobin in the case of most 

 worms and a greenish substance chlorocruorin in some others ; 

 a yellow pigment, echinochrome in the case of sea-urchins 

 and haemocyanin in that of higher Crustacea and of some 

 molluscs. Haemocyanin is colourless in the living animal, 

 but acquires a bluish tinge upon exposure to the air. The 

 respiratory pigment may be carried in special corpuscles 

 (amoebocytes), as in the case of the sea-urchin, or may be 

 simply dissolved in the blood plasma. 



Respiration nearly always involves a mechanical process 

 whereby a renewal of the respirable medium is ensured 

 to the animal, the degree to which such a process is necessary 

 being governed by the extent to which the organs of respira- 

 tion are protected, by the sluggish or active habits of the 

 animal, by the nature of its normal habitat, and so on. In 

 a typical land vertebrate, owing to the enclosed situation of 

 the respiratory organs, which is necessary in order to 

 avoid desiccation, the active nature of respiration is very 

 marked. The renewal of the air is effected by the rhyth- 

 mical contraction and expansion of the respiratory organs 

 themselves, aided by the movements of the chest wall. 

 In insects the same object is attained by the opening and 

 closing of the stigmata or entrances to the air-tubes, and by 

 the compression of the air-tubes themselves. In aquatic 

 animals, particularly in those forms in which the gills are 



