LOWER INVERTEBRATES WHICH BREATHE IN WATER 417 



In the Organ-pipe Coral {Tiibipora inusicd), for instance, each 

 member of the colony lives in a tube, from which only the mouth- 

 end can be protruded, and it is the external surface of this end 

 which probably does most of the breathing. The eight broad 

 feathery tentacles which surround the mouth largely increase the 

 available area, and they are beset with cilia which set up currents 

 by which the water in contact with them is constantly renewed. 



Examination of fig. 416, p. 162, which represents one of the 

 free-swimming Compound Jelly- Fishes {Physophord), will show 

 that the numerous and diversely-shaped members of which the 

 colony consists present collectively a very large external surface 

 over which breathing may take place. At the top is a float, below 

 this a stalk-like part on which are arranged numerous swimming- 

 bells, and under these come a circlet of leaf-shaped structures, 

 which cover feeding individuals, from which numerous long 

 branched fishing-lines trail in the surrounding water. 



It must further be remembered that the great diversity of 

 animals included in the group of zoophytes are to all intents 

 and purposes living stomachs of more or less complexity, and, 

 since a great deal of water is taken in with the food, the lining 

 of the large internal digestive space is able very materially to 

 help on the respiration. Indeed it may be remarked generally 

 that in the lower groups of the animal kingdom division of 

 physiological labour is not effected to anything like the same 

 extent as in the higher groups. 



In some of the Sea-Anemones 

 there is a special arrangement pro- 

 moting very greatly the internal 

 breathing above described. For, as 

 in all zoophytes, the mouth is not 

 precisely equivalent to the aperture 

 so named in, say, an earth-worm, for 

 it serves not only for the taking in of 

 food, but also for casting out such por- 

 tions of this as have not been digested. 

 It is of slit-like shape, and leads into a gullet along which run a 

 couple of ciliated grooves, one beginning at each corner of the 

 mouth. Except when large prey is being swallowed or large 

 undigested fragments passing out, the sides of the gullet are in 

 contact and the central part of the mouth-slit closed. The ciliated 



Fig. 546. — Diagrammatic vertical section of 

 a Sea- Anemone, showing ciliated grooves of 

 gullet. Arrows show the way in which currents 

 of water flow in and out of the slit-like mouth. 



59 



