DISTRIBUTION IN SPACE. 169 



In some places it is rocky, in ollicrs saiuly or niiiddy, or covered with dctaclied blocks or with 

 old shells. It possesses but a scanty vegetation, and this consists chiefly of the red alga,'. 

 Hydroid life, however, would seem to attain here its niaxiuuini of development. The sandy and 

 muddy bottoms possess but few species, while our knowledge of the hydroid tenaflts of the 

 rocky ground is still dcticient, in consequence of the difficulties here experienced in ex|)loration 

 by the dredge, and our acquaintance with them is chiefly derived from such as have been brought 

 up entangled in the lines of the fishermen. All the more minute forms which attach themselves 

 to the rocks must necessarily escape this mode of capture, and it is highly probable that the 

 rocky bottoms of the coralline and other deeper zones abound with an unknown hydroid fauna, 

 the source probably of many of those free-swimming planoblasts which have not yet been traced 

 to their trophosomes. 



The stony and shelly bottoms, on the other hand, offer great facilities for the use of the 

 dredge, and from none of the zones has such an abundant hydroid fauna been procured. 

 The following species may be enumerated as among the most characteristic : 

 Corymorpha nufans, Hydractinia echlnata, Endendrium ramosum, Periyonimus serpens, Peri- 

 gonbmis mimdus, Bougainvillia ramosa, Bicoryne conferta, Obelia longissima, Laomedea vohbilis, 

 Laomedea verticillata, Copphiia arcfa, Lafoea dumosa, Haleciiim Jialecinum, Sertularella polyzonias, 

 Sertularia fdicula, SertuJaria aryoitea, Serfidaria abietina, Diphasia tamarisca, Thuiaria thuja, 

 Hydrallmania falcata, Antennidaria antennina, Plumularia setacea, Plumularia pinnata. 



V. The Deepwater Zone. — The Deepwater Zone extends from the lower limit of the Coralline 

 Zone to a depth of about 100 fiUhoms from the surface. The Htdroida, as we have just seen, had 

 attained their maximum of development in the Coralline Zone, and we now find them begin to 

 decline in variety of form and in multitude of individuals. The plant-life, with the exception of 

 Diatomacea and some nullipores, has disappeared. There is tlie same variety of ground in this 

 zone as in the Coralline, but the muddy bottoms with their scanty hydroid life are more frequen*. 

 For our knowledge of the species frequenting the rocky bottoms of the Deepwater Zone we are 

 also mainly indebted to the long lines of the fishermen. 



Most of the species, however, which occur in this zone have also been found in others. 



The following are deepwater species, and will give a fair notion of the character of the 

 hydroid fauna of the zone : 



Tubularla simplex, Tubularia attcnuata, Merone cornucopiee, Lafoea fruficosa, Grammaria 

 abietina, Haleciiim muricatum, Halecium labrosimi, Sertularella Gayii, Sertularella tricuspidata, 

 Diphasia pinaster, Thuiaria thuja, Thuiaria articulata, Aylaophenia myriophylltnn, Plumularia 

 catharina, Plumularia frutescens. 



VI. The Abyssal Zone. — This zone commences at the lower limit of the Deepwater Zone, and 

 extends to the greatest oceanic depth yet determined. Our knowledge of its faunas is of quite 

 recent acquisition. We are mainly indebted for it to the deep-sea dredgings which have been 

 lately carried on in the North Atlantic, and which have formed so important an era in zoological 

 research. Its bottom would seem to vary like that of the two preceding zones, though great tracts 

 of fine mud loaded with Globiyerince and other protozoal forms, would seem to constitute the 

 most widely extended and characteristic grounds. With the exception of Diatomacea, plants 

 are entirely absent ; and even the Diatomacea disappear in the lowest regions of the zone. 



