158 PRIMITIVE FORMS OF LIFE 



as we have seen, the main forms of the pelagic and the ichthyo- 

 logical fauna of the deep sea. These two invasions of the fresh 

 waters were followed by a third. This time the newcomers 

 had soft dorsal fins, pelvic fins near the pectoral, and a closed 

 swim-bladder. This kind, however, is still scarce, and is 

 only represented in European fresh waters by the Lote, which 

 are related to the Cod. Certain others belong to the 

 pelagic forms that have a partially spinous dorsal fin and 

 include strong swimmers related to the Perch. Some 

 of these, like Coitus and the Red Gurnard, had reverted to 

 their littoral habitat, and were therefore predisposed to enter 

 fresh water. The Chubs of our rivers belong to this group. 



Many marine Fish spawn an innumerable quantity of small 

 eggs and abandon them without bestowing the slightest care 

 upon them. As a rule, however, the species that penetrated 

 into fresh water and remained there belong to genera or 

 families which produce but a few large eggs, and attach them 

 to the under side of stones, to algae, or inside empty 

 shells, if they do not actually spawn in shelters prepared 

 in advance. These eggs are large because they are filled with 

 nutritive substances which save the embryo from seeking any 

 other nourishment until the supply is exhausted. Under these 

 conditions the embryo grows very rapidly, and when it leaves 

 the egg still often carries a part of the reserves in a receptacle 

 called the vitelline sac. Thus it acquires both the agility and 

 the resistance that will enable it to escape many of the dangers 

 besetting it. This enhancement in the size of the eggs can also 

 be observed in the case of the Prawns, which penetrate fresh 

 waters and are hatched in a form that is almost mature, while 

 their congeners still have profound transformations to undergo. 

 The same difference exists between the Sea Crayfish and the 

 Lobster. The former have small eggs giving rise to transparent 

 swimming embryos called Phyllosoma, which in no way 

 resemble the adults ; the others, on the contrary, spawn large 

 eggs from which the young are hatched out in their permanent 

 form except that they have still to grow, and it is probably 

 this that has permitted kindred forms to penetrate into the 

 fresh waters where they have engendered the diverse forms 

 of Crayfish. 



The instability of the conditions of existence in fresh water 

 appears to have resulted among the invertebrates which sought 



