PEOPLING OF LAND AND SEA 165 



particularly competent in all that appertains to the history of 

 Worms, classed genuine Leeches, such as Branchiobdella 

 among the Naidids. The most salient leech characters appear 

 in certain Central African Worms, the Polytoreutidae, 1 in which, 

 as in Leeches, the reproductive orifices are placed in a median 

 ventral line instead of according to the usual symmetrical 

 arrangement. In becoming carnivorous or parasitic, Leeches 

 have merely continued to inherit the hermaphroditism of their 

 oligochaete ancestors, who acquired it when they took to life 

 in marine or freshwater lakes. Subsequently they trans- 

 mitted the character to the Trematodes, which, when they 

 became free organisms, gave rise to the Turbellarians. 



We have an even simpler explanation of the presence of 

 Opisthobranchs, which are all hermaphrodites, in the seas. 

 Like the Pulmonata, they have lost their primitive gills. This 

 loss suggests that the ancestral Opisthobranchs at one time left 

 the water and lived in the open, or at least in a low-lying littoral 

 zone washed by the tides, and thus often out of the water for 

 long periods. In this state they still remain, except for their 

 derivatives, the pelagic Pteropods. Had they remained aquatic 

 they would have preserved their branchial apparatus. There is 

 no reason why they should lose a respiratory system so 

 eminently advantageous after having once acquired it. It 

 must therefore have been during their change of habitat that 

 they became hermaphrodite like the Pulmonata, which aiso 

 lost their gills and present so many characters analogous to 

 the Opisthobranchs that we may justly ask whether some 

 phylogenetic relationship does not exist between these two 

 orders, and whether they are not linked up by certain still 

 existing non-aquatic forms. 2 Having reverted to their earlier 

 environment the re-developed gills around the anus, 3 on the 

 back, 4 on one, 5 or on both sides of the body. 6 



Access to dry land was not so easy as might be imagined. 

 In the first place there had to be preparation, and this, which 

 can be regarded if one so wishes as a pre-adaptation, had else- 

 where always consisted in the disappearance of the external 



1 LXXII. 



2 Oncidium. 



3 Doridae. 



* Nudibranchs. 



5 Umbrellidae, Plcurobranchs, Aplysia. 



6 Phyllidiae. 



