LOWER INVERTEBRATES WHICH BREATHE IN WATER 411 



(see p. 243), the current-producing organs in either case being a 

 more or less complex plume in the neighbourhood of the mouth. 

 These organs are no doubt also effective for breathing purposes. 

 In Moss- POLYPES something of the kind is clearly necessary, for 

 although in these colonial animals each individual is of small 

 size, and might perhaps be expected to breathe by means of the 

 general surface, a large part of this is prevented from doing 

 respiratory work. For every member of the colony is to a great 

 extent enclosed in a horny cup, which is an effectual hindrance 

 to exchange of carbonic acid gas for oxygen, and the soft part of 

 the body which projects from the cup would not give a sufficiently 

 large breathing surface were there not special outgrowths from it 

 (see p. 261). 



LAMP-SHELLS are solitary animals of much larger size than 

 the individual members of Moss- Polype colonies, and it is there- 

 fore not astonishing that there should be outgrowths from their 

 bodies which serve as gills, though (as in Moss- Polypes) these 

 outgrowths play a double part. Nor must it be forgotten that 

 the body of one of these animals is covered and protected by 

 a firm bivalve shell, which in one sense diminishes the surface 

 available for breathing. This, however, is compensated by the 

 fact that each half of the shell is lined by a thin flap of the 

 body-wall which takes part in the work of respiration. Although 

 Lamp- Shells are but very distantly related to Bivalve Molluscs, 

 there is a curious similarity between the two groups as regards 

 feeding and breathing (see p. 248). This is only one of very 

 many cases in the animal kingdom where more or less similar 

 organs have been independently evolved in quite distinct groups, 

 which have had, so to speak, the same physiological problem to 

 solve. 



ECHINODERMS (ECHINODERMATA) 



This large, important, and very sharply limited group or 

 phylum of the animal kingdom includes Sea- Lilies (and Feather- 

 Stars), Sea-Urchins, Star-Fishes, Brittle-Stars, and Sea-Cucumbers, 

 besides other forms which have long been extinct. Two char- 

 acteristics of these animals are especially noteworthy, as they 

 largely influence the nature of the breathing organs and mode of 

 breathing. They are: (i) A strong tendency to develop a firm 

 protective armour, consisting of limy plates in the skin, often 



