326 



LIFE BY THE SEASHORE. 



to find that it is so ambiguous that it is capable of inter- 

 pretation in two diametrically opposite ways. It may be 

 affirmed that (1) pelagic animals 

 have arisen from littoral ones, 

 and (2) littoral animals from 

 primitive pelagic forms, and both 

 positions can be supported by an 

 imposing array of arguments. 

 Think of the life-histories of the 

 littoral animals we have studied : 

 In the Coelentera we have often 

 an alternation of generations, the 

 life-history including a jelly-fish 

 stage adapted for a pelagic habi- 

 tat, and a fixed zoophyte stage 

 adapted for life on the bottom. 

 Among the worms there is usually 

 a larval pelagic stage ; a little 

 top-shaped larva called a trocho- 

 sphere occurs in the life-history 

 of most of the marine bristle- 

 worms, and is to be found near 



the surface of the sea '. swims 

 should be contrasted with Fig. 6. by means of the motile threads, 



Note short manubrium and the ! -_i,' v. V* A 



four tentacles. After Hincks. or cilia, which occur in bands 

 on the surface of the body, and 



is later, by a process of metamorphosis, converted into the 

 more or less sedentary adult. The Echinoderms, again, as 

 we have already seen, have larvae very different in character 

 from the adults, and adapted for a free swimming and not 

 a sedentary existence. We have also emphasised the occur- 

 rence of pelagic larvae of many strange shapes among the 

 Crustacea, and a tow-netting at almost any season of the 

 year will show you that the surface-water simply teems 

 with these. The Mollusca also add their quota of minute 

 larval forms to the fauna of the open sea. Generally we 

 may say that although there are a few exceptions, yet it is 

 true of littoral animals as a whole that they produce minute, 

 active, pelagic larvae. 



Further, these larvae are usually simple in structure, and 

 are often devoid of those peculiarities which are diagnostic 



