478 ESSENTIALS OF ZOOLOGY 



Means of Dispersal and Barriers 



The wide distribution of species of animals on the earth depends 

 largely upon their means of dispersal or means of being carried from 

 one place to another and upon the barriers which they encounter. 

 Among the members of a species as well as among the related and 

 nonrelated species of animals there is a continuous struggle for 

 existence. Those forms which, by some means, are able to enter new 

 environments where competition is less severe will have the better 

 chance at survival. 



In practically all marine organisms there exists a means of loco- 

 motion during some stage of the life history. In a great number 

 of the forms, especially those which are sessile in the adult stage, 

 there is a free-swimming larval stage: planula larva of the coelen- 

 terates ; trochophore larva of annelids ; and the various free-swim- 

 ming larvae of mollusks, echinoderms, and crustaceans. The plank- 

 tonic larvae and adults, i.e., those which swim or float free of the 

 bottom, depend greatly upon oceanic currents for their wide dispersal. 



The dispersal of oceanic animals is also greatly enhanced by the 

 large numbers of eggs and larvae produced by these forms. For 

 example, one investigator found that a **sea hare," a marine gas- 

 tropod, deposited 478 million eggs during one four-month spawning 

 period. The eggs were laid at the rate of 41,000 per minute. 



Marine animals are limited in their distribution by such barriers 

 as temperature, land masses, and salinity of the water. The marine 

 animals on each side of the narrow Isthmus of Panama are entirely 

 different. The extreme changes in temperature prevent most of 

 these species going around the southern tip of South America. Lit- 

 toral animals are often limited in their distribution by large river 

 mouths which empty great quantities of fresh Avater into the oceans. 

 Only specially adapted forms can live in these brackish waters. 



Fresh-water faunas are restricted in their distribution by land 

 barriers which usually separate the bodies of Water in which they 

 live. These forms depend mainly upon other animals, such as birds 

 and insects, for the dispersal of their eggs and dormant stages. 

 Many fresh-water animals, such as snails and clams, attach them- 

 selves to the bodies of birds or insects and are carried into new 

 habitats. 



Among land animals the birds and flying insects appear to be 

 least restricted in their ranges. Even these, however, are often 



