333 



is to a large degree dependent for food on those organisms 

 which are swept within its reach by tlie currents, while the 

 vagrant benthos becomes more actively engaged in seeking 

 out its food. A large number of sedentary benthonic ani- 

 mals have assumed a radial structure — especially well typi- 

 fied in corals and crinoids, and also shown in the corona of 

 the barnacle; while others, such as brachiopods, have a 

 bilateral symmetry of high degree. Some of the lower 

 vagrant benthonic animals, e. g., echinoderms, are also built 

 on the radial plan, but the majority of free benthonic ani- 

 mals are bilaterally symmetrical. Among the vagrant ben- 

 thos the struggle for existence is most intense, and, as a 

 result, the variety of adaptations and the wealth of form 

 and color is almost unlimite<l. Transitions from the vagrant 

 benthos to the nekton are numerous, and it becomes some- 

 times difficult to decide if an animal belongs to the vagrant 

 benthonic or to the neytonic type. The gradation is just as 

 complete as between nekton and plankton. In general a 

 radial form may be said to be characteristic of the sedentary 

 benthos, while a bilaterally symmetrical form is as charac- 

 teristic of the vagrant benthos. Examples of change of form 

 with change of habit occur in many classes. 



The term mero-plankton, from fxepo^, a part, was also in- 

 troduced b}' Hfi?ckel, and is applicable to the larvte of ben- 

 thonic animals which lead, during the larval stages, a truly 

 planktonic existence, and which occur with, and suffer 

 the same vicissitudes as, the true or holo-plaukton. The 

 upper sti-ata of the ocean are coiumonly crowded with such 

 mero-|)la]iktonic organisms, and to them is due the hori- 

 zontal distribution of benthonic species. Floating about 

 in the sea, in perfect clouds or swarms, these mero-plank- 

 tonic organisms pass their short existence, a s|iort of the 

 waves and currents. Sooner or later, how'ever, they will 

 sink to the bottom, a veritable rain of seedling organisms, 

 and if they fall upon a fertile soil, in other words, if 

 they reach the proper facies of the substratum, they will 

 develop into the benthonic adult; but if they fall upon an 



