COELENTERATA. 169 



fishes modified into (1) a float supporting the colony; 

 (2) swimming-bells by means of which it moves; others (3) 

 for feeding, still others (4) for digestion, and again others 

 (5) for reproduction. In all of these only that part of 

 the medusa which is necessary for the function is retained. 

 Thus in the figure it will be seen that the swimming- 

 bells have no proboscis, the feeding individuals consist 

 of proboscis alone, etc. Usually one or more of these 

 modified medusae is lacking from the colon}'. The most 

 familiar of the Siphonophora is the ' Portuguese man-of- 

 war' (fig. 16), which occasionally drifts on our shores. 

 In this beautifully-colored species the float is large and 

 the swimming-bells are absent. 



CLASS II. SCYPHOZOA (Sea-anemones, Corals, 



Medusae, etc.). 



While the Hydrozoa may be likened to a double bag, 

 the two bags corresponding to the ectoderm and entoderm 

 (p. 154), the Scyphozoa, fig. 17, might be compared to the 

 same bags, with the mouth turned inwards, and extend- 

 ing as a tube for some distance into the interior. In 

 this way there arises a short food-tube or oesophagus lead- 

 ing from the mouth to the digestive cavity. The diges- 

 tive cavity itself is subdivided by partitions or septa 

 radially arranged so that there is a central chamber and 

 connected with it chambers between the septa. To these 

 characteristic features may be added others. Thus there 

 is a circle of (usually hollow) tentacles surrounding the 

 mouth, and the edges of the septa bear long thickened 

 threads, the mesenterial filaments, which are digestive in 

 function. The septa greatly increase the surface of the 

 digestive cavity so that the food dissolved by the secre- 



