178 REPRODUCTION 



instinct to get at the eggs. Once they are hatched, the parental 

 care is at an end, and the young fish fend for themselves. 



In many other fish, such as the sea-horse, the lump- 

 sucker, etc., and certain frogs, the males guard and tend the 

 eggs of their mates. In fact, amongst the fauna of the sea- 

 shore paternal care is as common, if not more common, than 

 maternal. But in land-vertebrates it is the mother as a rule 

 that rears, tends and fosters her offspring. 



A very large number of water animals and plants have 

 their eggs fertilized whilst floating freely in the sea or fresh 

 water ; the antherozoids of the latter and the spermatozoa of 

 the former are adapted for swimming through an aqueous 

 medium. 



Life originated in the sea, and certain considerable groups 

 of animals have never left it. In spite of deserters, the marine 



Fig. 66. Amphioxus lanceolatus from the left side, about twice natural size 



After Lankester. 



fauna is still far richer and more varied than that of the fresh 

 water or of the earth. In fresh water and on the earth there 

 are none of those flinty little Radiolarians which form so large 

 a part of the sea bottom, nor are there calcareous Fora- 

 minifera in our streams and lakes. Chalky and horny sponges 

 exist only in the sea, and many of the larger sub-groups of the 

 Coelenterata do not penetrate fresh waters, though occasion- 

 ally a jelly-fish is found. Curious worm-like animals known as 

 Gephyrea and Chaetognatha are found only in the ocean. 

 Many large groups of theMoLLUSCA, prominent amongst which 

 are the cuttle-fishes and the Pteropoda upon which the young 

 whales so largely live, are exclusively marine. No specimens of 

 the groups Echixodermata, starfishes, sea-urchins, sea-lilies, 

 occur outside the sea, and one geologically extremely old group, 

 the Brachiopoda, are all purely marine in their habitat, and 

 so are the little forerunners of the Vertebrata, the Ascidians 

 and a primitive little fish-like form known as Amphioxus, The 



