363 



occurring below 400 fathoms. A number of species live 

 in fresh water or on land. The majority of decapods leave 



the egg in the Zoea stage, in which the abdominal region is 

 perfectly segmented, though still without appendages, 

 except perhaps the rudiments of the sixth pair. The com- 

 pound eyes are stalked. Other larval stages follow until 

 the adult is reached. These larva* often occur in vast 

 quantities in the plankton. 



Migration of Marine Invertebrates. 



Whatever the present distribution of animal life in the sea, 

 it is clear that it has not always been so. Even the most 

 cosmopolitan species had its circumscribed center of origin, 

 it being extremely unlikely that the same species originated 

 in more than one locality. From this locality the species 

 dispersed to occupy whatever territory was available. 



"The species," says Wagner,* " which originate in isolated 

 areas, seek every opportunity to occupy new regions, and 

 migrate from their place of origin to other localities.*' 

 However, as Ortmann insists, species can only occupy areas 

 the fades of which correspond to those of regions where 

 they originated, in the degree in which the species is depend- 

 ent upon the fades. Thus, while emigration from the orig- 

 inal home of the species may take place on a large scale, 

 immigration and settlement of this species in other regions 

 is only possible under certain circumstances. 



It must not be forgotten in this connection, that, as al- 

 ready pointed out, the organic factor is of foremost impor- 

 tance in determining whether or not an area is to be per- 

 manently occupied by a new immigrant. If the food supply 

 is insufficient, or if contending species hold the ground, the 

 new arrivals maybe prevented from occupying the territory, 

 even though the facies is well adapted to their needs. 



Continuity of the conditions of existence, in space, favors 

 dispersion, for in a continuous area there are no barriers 

 to surmount. On the other hand, discontinuity of the 



* Ortmann, '95, p. 33. 



