INTRODUCTION. xlvii 



in the Scame group, sometimes, as in the case of Balanoglossus, even in the same genus, 

 one species will develop directly, while another has a complicated life history, lie will 

 be led to the inquiry, What is the meaning, what the use of this metamorphosis in 

 one and not in the other? If one examines carefully the embryological changes of 

 those forms hatched in the form of the adult, he will see that frequently they present, 

 while in the egg, an epitome of the develojDinent of their relatives in whicli the changes 

 have been much more marked. The question is largely one of nutrition, though many 

 other conditions enter into the problem. In those forms where the food supply in the 

 egg is abundant, the tendency is to simplify the development and to accelerate it. All 

 superfluous features are consequently omitted, or are passed in a hasty manner. On 

 the other hand, where the amount of food is small, the animal is forced to begin life 

 for itself at an early date, and hence it needs every protection against the dangers 

 of its environment. 



An interesting point to be noticed in this connection has recently been described by 

 Mr. W. J. Sollas, in the development of the sponge, Halisarca. In the Mediterra- 

 nean the embryos of this sponge escaj)e from the tissues of the parent when they 

 have arrived at the blastula condition, and tliey then swim about freely by means of the 

 cilia clothing the surface ; in the same species on the shores of the English Chamiel, 

 the young are retained until after gastrulation and the formation of the canal system. 

 According to Sollas the explanation of this difference is not difficult. In the Medi- 

 terranean there are no strong currents, and it is evidently best for the parents to get 

 rid of the young at as early a moment as possible, thus escaping a longer drain upon 

 its energies. In the English Channel, on the other hand, the current is very strong, 

 and were the embryos to be set free at the stage at which they are in the Mediterra- 

 nean, the chances are that they would be swept away from proper places for their 

 further development, and hence they are retained until nearly ready for attachment to 

 the rocks. 



The same influences, nutrition and environment, affect other forms. Almost all 

 crustaceans undergo a complicated metamorphosis, and in their various stages they 

 lead very different lives. In the young they are usually free-swimming, and hence 

 they need protection from aquatic foes. This is usually gained in two ways ; by trans- 

 parent tissues which render them invisible to fishes, and by the development of spines 

 and processes from the body, which increases their size without materially adding to 

 their weight, thus preventing their entrance to the mouths of the smaller fonns. Still 

 not all the Crustacea undergo these changes ; in the whole group of tetradecapods no 

 metamorphosis is known, while in the land-crabs of the tropics the young, when 

 hatched, are closely similar to the ])arents. In this latter instance, where the adults 

 live on the land, only going to the sea at the approach of the breeding season, it is easy 

 to be seen why the development should be direct. 



In other cases the use of larval forms is very evident. Many forms, like the l)ar- 

 nacles, sponges, and the oysters, lead a stationary life, but the young are free-swim- 

 ming. This change in form and mode of life undoubtedly is of great benefit to the 

 species, for if at a given moment the parents were swept out of existence, the young, 

 living in a different station, would continue the species; and, besides, they serve to 

 distribute the race from point to point. 



The foregoing paragraphs have reference to the larval forms, and the persistence and 

 value, and the benefits of a metamorphosis. Some of these characters are engrafted on 

 the primitive stock, while others are due to the origin, the evolution of the group, and 



