DISTKIBUTION BY CUKEENTS. 279 



animal world was remarkably poor in species, and more par- 

 ticularly was it devoid of all the larger forms, as Medusa, 

 swimming Polyps, Ascidians, and such like ; quite suddenly tho 

 ocean teemed with abundant life as we entered on the warm 

 Mozambique current to the east of the Cape. The whole sea 

 was literally covered with animals of every kind, and for more 

 than two days we sailed without interruption through fields of 

 gigantic Pyrosomata {P.giganteum, see fig. 71), which swam so 

 close together that at night the whole ocean, out to" the farthest 

 horizon, shone with their blue gleam as if in broad moonlight. 



Now we might easily be led to suppose that the winds must 

 act exclusively on flying or land animals, and currents, on the 

 other hand, exclusively on creatures living in the water. But 



!FlG. 71. — Pyrosoma gigas, 



the investigation of certain special cases will prove, on the con- 

 trary, that these influences are often combined, so that land 

 animals or air-breathers become dependent on currents in water, 

 and also aquatic animals on the winds. It is nevertheless my 

 intention to consider the action and effects of winds and cur- 

 rents separately. 



(a) Currents as a means of distribution. — By far the 

 greater number of invertebrate animals freely swimming or 

 floating in water are incapable of offering any resistance to the 

 current, and are therefore carried along in the direction which 

 the current itself takes. All the larvse of Sponges, Polyps, 

 Annelida, Tunicata, Echinodermata, and very many Mollnsca 

 as well as fully-grown Radiolarise, the floating Tunicata, Ptero- 

 poda, Heteropoda, many Annelida, and the MedusiB, though 

 many of these are provided with special swimming organs, are 



