196 University of California Publications in Zoology. [VOL. 4 



the surface and migrate to new areas. It is true that occasionally 

 the proboscis is protruded above ground, but with the rising tide 

 this is again withdrawn. Careful observations have failed to 

 discover any instance where the animal's body is in any way 

 exposed after the ground is covered with water. (Ritter '02, 

 p. 255.) 



It is also very probable that any individual animal is restricted 

 to a comparatively small area ; that it does not migrate from place 

 to place in the mud. This is indicated by the fact that burrows 

 are vertical for the most part, and that the burrowing movements 

 'are slow compared, for example, with those of certain nemertians 

 and annelids. Furthermore, the adult animals are not uniformly 

 distributed over the area in which they are found. In some 

 places as many as fifty individuals may be found in an area of 

 one square foot, while in others but one individual may be found 

 in an area of a square yard. 



The distribution of D. pusillus during a period of six years 

 at San Pedro, California, is described as follows : * ' The area of 

 greatest numbers has gradually shifted toward the mouth of the 

 harbor. None were found in 1902 in places where they were 

 abundant in 1897. In 1900 there were two areas of distribution, 

 one at low tide-mark, where large individuals predominated ; the 

 other considerably above low tide-mark, where small ones pre- 

 dominated." (Ritter-Davis '04, p. 200.) 



Here are two points worthy of notice. The shifting must not 

 be understood as migration in the sense of individuals moving 

 toward the mouth of the harbor, but rather as plants migrate. 

 For various reasons, among them being deposition of sediment, 

 change of currents, and invasion of eel-grass, the region populated 

 in 1897 became unfavorable for enteropneust life, and the animals 

 died. The opposite end of the mud-flat strip which was the 

 center of distribution in 1902 became so by the growth of new 

 individuals. In 1900 the increased number of new individuals 

 was great enough to be noticeable but in another direction of 

 distribution. Here they appeared farther shoreward. 



Before discussing means of dispersal for D. pusillus it will be 

 worth while to consider more in detail the environment of the 

 adult. The animal is always found in sheltered places, free from 



