DISTRIBUTION IN SPACE. 167 



cliiuatc. Wliilc the explorations of the dredge have revealed to us the fauna of other zones, this 

 has yielded up its treasures to the towing net. 



The hydroid fauna of the Surface Zone consists of the multitudes of planoblasts which have 

 been liberated from the fixed tropliosomes of other zones, and of such hydroid medusas as may 

 have been directly developed from the egg, all finding here conditions suited to their love of sun- 

 light and their great powers of locomotion. 



A few tropliosomes, however, which root themselves to floating seaweeds, may perhaps l)e 

 regarded as belonging to the fauna of the Surface Zone, just as some exceptionally formed plano- 

 blasts would seem to frequent other zones. To the latter belong Clavatella and Eleufheria, 

 which are found creeping over the seaweeds and nullipores of rock-pools in the Literal Zone, 

 and probably also Cladouema, which, besides its power of swimming like other medusas, has also 

 the faculty of mooring itself to fixed bodies.^ 



The Surface Zone is not without its plant-life, but its flora is a floating one — the gulf-weed 

 of the Sargasso Sea {Sar(/assum hacci/erum), and the lower Algse — Oscillatorm and Biatomacece, 

 which often accumulate in such quantities, even round our own shores, as to impart their colours 

 to the sea for many square leagues. 



With the hydroids of this zone is associated a rich pelagic fauna of Badiolaria and other 

 Protozoa, of discopliorous raedustc, and of siphonophores and ctenophores, as well as of pelagic 

 molluscs — pteropods and heteropods ; and pelagic ^^«//«/o«« — Sar/itta and Tomopteris ; and the 

 larval forms of echinoderms and annelidans, and of molluscs, which in their adult state 

 creep over the ground or moor themselves to the rocks. Certain fishes, too, must be included in 

 it, for many species have their habitual abode in the Surface Zone, and pursue their prey in this 

 highest region of the sea. It is the zone of sun.shine, the region where life becomes intensified, 

 and where beings whose organisation specially fits them for the enjoyment of the more exciting 

 influences of the external world find a congenial dwelling-place. It is the zone, too, where phos- 

 phorescent animals congregate in countless multitudes, and light up at night with their mysterious 

 fii'es the dark surface of the sea. 



II. Tlie Liforal Zone. — The Litoral Zone constitutes round oui' shores a well-marked belt. 

 It extends through the entire space which exists between the flood and the ebb levels of ordinary 

 tides. Among the hydroids which occur in it are some of those species which are the most 

 decidedly limited in their bathymetrical range. Of the various species which constitute its 

 hydroid fauna some will be found rooted to the moist rocks which have been left uncovered by 

 the retiring tide, or may be seen spreading over the fronds of the ohve-coloured Algae — Fucus 

 vesiculosus, F. nodosus, and F. serratm — which form the chief features in the vegetation of 

 the zone ; while others have their favourite abode in the rock pools with which this zone 

 usually abounds, and in which they are associated with hundreds of living beings belonging to 

 very different groups, all findhig like the hydroids a congenial abode in the clear waters of the 

 rock-pool. 



The greater part of the species, however, which are peculiar to this zone, are not found in the 



' The curious hydroid forms, Nemopsis and Acaulis, have been taken by the towing net in this 

 zone; but these, as we have already seen, are probably only the detached hydranths of tropliosomes 

 routed to the bottom of some of the deeper zones. 



