Distribution of Marine Animals. 127 



tributed in the ocean at a former period, but that physical 

 changes had subsequently restricted the range. Others see 

 in this condition proof of the enormous powers of dispersion 

 at the disposal of marine organisms, and the origin, in the 

 several areas, from a pre-existing form. 



Moreover, whatever may be the conditions (and Sir J. 

 Murray thinks the quantity of carbonate of lime secreted by 

 marine organisms is determined by the temperature of the 

 water and therefore chiefly chemical rather than physical) in 

 regard to coral-reefs, northern Annelids (e. g. Filigrana 

 implexa and other Serpulidse) have no difficulty in forming- 

 considerable masses of calcareous tubes *. Temperature 

 appears to have no appreciable influence on the abundance 

 and size of these calcareous tubes in cosmopolitan species. 

 Nor is there a distinction in regard to the calcareous secre- 

 tions of the Polyzoa and Echinoderms of the extreme north 

 from those in the tropical oceans. 



The families o£ the Nemerteans have a range as wide as 

 that of any previous group, and the type of structure varies 

 little whether the form be arctic, tropical, or antarctic. Of 

 no special region of the ocean can it be said that its Nemer- 

 tean fauna is diagnostic, for with advancing knowledge 

 (largely due to the labours of Mr. R. C. Punnett) the dis- 

 tribution of the types is always extending. There is no 

 evidence, moreover, that the arctic and antarctic forms have 

 other relationships than those which spring from a cosmo- 

 politan distribution. 



So far as can be ascertained, the families of the Echino- 

 derms correspond with those of other groups in regard to 

 distribution. Some range from the arctic to the antarctic 

 seas, and, as Mr. Bather observes, from the eastern shores 

 of America round the world to the western, the same 

 species thus occurring on the opposite shores of the Isthmus 

 of Panama. It has, however, to be remembered that a com- 

 munication existed between the respective sides up to a 

 recent period. Some, again, range to great depths as well as 

 have a wide distribution. 



As in other groups, some forms suggest a northern area 

 and some a tropical, but on the whole it cannot be stated 

 that there are special regions of the ocean characterized by 

 special families of Echinoderms, though it is true that certain 

 types, like the Pentacrini and Elasipoda, occur in deep water. 



* Murray thinks that those forms secreting- a large quantity of car- 

 bonate of lime would be killed by the lowering- of temperature at the 

 poles — like those with pelagic tarvse, 



