166 PRIMITIVE FORMS OF LIFE 



apparatus of aquatic respiration, of which traces sometimes 

 persist, a disappearance that has often been produced in 

 immigrant marine organisms when they took up life in fresh 

 waters. For this aquatic respiratory apparatus was substituted 

 an internal one, which was thus protected against desiccation, 

 a danger to which air-breathing animals would be constantly 

 exposed, but which a marine animal need not fear as its 

 respiratory organs are always submerged and have only to be 

 protected against collision, or the predatory attacks of small 

 carnivorous creatures. Occasionally the branchiae, which 

 constitute the pre-eminent aquatic respiratory apparatus, were 

 not replaced, the surface of the body sufficing for aeration. This 

 is what happened in the case of the Earthworm, their close 

 kin the freshwater Annelid Worms, and the Leeches. Among 

 those organisms which live in fresh water or have returned to 

 the sea, the branchiae in certain conditions can be redeveloped, 

 as in the Opisthobranch Gasteropods. Thus, those beautiful 

 little Freshwater Worms, Dero (LVIII), have a sort of 

 outgrowth at the posterior extremity of the body, supporting 

 four retractile finger-like processes, the whole constituting a 

 respiratory mechanism over whose surface the water is 

 constantly renewed by the action of powerful waving cilia. 

 In the same way Ozobranchus, a Leech which lives in the mouth 

 of Crocodiles, marine Tortoises, and Pelicans, and the 

 marine Leeches of the genus Branchellion, living on the Electric 

 Eels, have recovered these branchiae, in the first case in the 

 form of tufts, and in the second in the form of trumpets. 



The substitution of an internal for a branchial respiratory 

 apparatus naturally consisted merely in a process of 

 invagination of certain portions of the integument, or in 

 the adaptation to a respiratory function of internal organs 

 having communication with the exterior, as is the case with the 

 digestive apparatus. By means of a new application of the 

 principle " everything happens that can happen ", the two 

 types have been arrived at by methods sometimes a little 

 unexpected, and, moreover, independently of the conditions 

 of the habitat. The larvae of Dragonflies, though they remain 

 exclusively aquatic, have an internal respiratory apparatus 

 contrived at the expense of the rectal region of the digestive 

 tube. This same rectal region, provided with powerful 

 vibratile cilia, constitutes in Freshwater Worms a supple- 



