1915.] Fauna of the Chilka Lake : Coe! enlevâtes . 75 



tion has produced definite structural changes : the former view seems to me the 

 more probable. 



At Port Canning and in the Chilka Lake examples of both Pelocoetes and 

 Phytocoetes may be found within a radius of a few yards. In both localities the 

 Pelocoetes will be deeply buried, at least up to the base of its oral disk, in dense mud. 

 At Port Canning P. gangeticus is most abundant in the canals of the sponge Spongilla 

 alba and in hollows on its surface, but is also found in abandoned burrows of Teredo 

 in the few wooden posts that exist in the pools, and occasionally quite free among 

 filamentous algae; in other parts of the Gangetic delta it occurs, often half -buried in 

 mud, on the roots of reeds. At Rambha P. chilkaeus occurs mainly among algae, 

 but there are neither Spongillidae nor worm-bored posts in those parts of the lake 

 in which it has been found. 



It is thus evident that while Phytocoetes has to some extent the habits of a young 

 Sagartiid, Pelocoetes has adopted a mode of life differing from that of any phase of 

 Metridium, indeed of any other allied form. All its generic peculiarities — its vermi- 

 form body, its reduced disk, even its incapacity to withdraw its tentacles — are 

 correlated with this mode of life, but apart from these features it retains the 

 structure of a Metridiine Sargartiid, and its basal disk is still functional, for if a 

 living individual is examined immediately after being dug out from the mud it 

 will be seen in most instances that the disk, small as it is, adheres to a particle of 

 shell or some other hard body. Although, therefore, the type must be regarded as 

 quite distinct from Metridium and Phytocoetes , I still believe that it is genetically 

 related to Metridium schillerianum, from which it has been evolved directly, most 

 probably in the Gangetic delta. Whether its evolution is due to natural selection 

 (i.e. to the survival of individuals that exhibited a slight tendency to burrow, and 

 of their offspring) or to mutation (i.e. the sudden appearance of a burrowing 

 strain, in the species) there is no evidence to prove ; the fact that Phytocoetes is 

 intermediate in structure between the two extreme types might seem to support the 

 natural selection theory, but there is as a matter of fact nothing definite to show that 

 the two new genera do not represent different offshoots from the main stem of 

 Metridium ; and if Phytocoetes is a permanent larval form it is difficult to imagine 

 it as an actual step in the ladder of evolution. 



My present views on these Metridiinae of Indian estuaries and lagoons, therefore, 

 may be summarized as follows : — 



(1) Stoliczka's Sagartia schilleriana is a Metridium. 



(2) The form I described in 1907 as a variety of M. schillerianum under the 



name exul is a distinct species and represents a new generic type, for 

 whicri the name Pelocoetes is proposed. 



(3) What I took for the young of this form represents a second new generic 



type, for which I now suggest the name Phytocoetes. 



(4) Phytocoetes is probably a permanent or quasi-permanent post-larval form 



of Metridium. 



(5) Pelocoetes is probably related genetically to Metridium schillerianum, but 



