iç)i5-] Fauna of the Chilka Lake : Echiuroidea. 63 



We have failed to obtain further specimens, but this does not necessarily mean 

 that the species is scarce, for, from the point of view of the collector, the Gangetic 

 mud is very difficult to deal with in a satisfactory manner. 



The specimen bears the number Z.E.V. 6807/7 m the Indian Museum books. 



T. dendrorhynchus , the Gangetic species, and T. sabinum — despite the specific 

 name of the last — all live in peculiarly dense mud, and we believe that the unusual 

 structure of the proboscis in the three species is correlated with this fact. It is 

 noteworthy that the gill-like outgrowths are better developed in the species from the 

 Gangetic delta than in that from the Chilka Lake, for the mud of the former region 

 is extremely fine and therefore forms a peculiarly dense and sticky mass. 



In most species of Thalassemia the proboscis seems to be the most active agent in 

 burrowing or in insinuating the body into crevices, but apparently this is not the case 

 in the three mud-living species just discussed. In these species the excavations are 

 formed by movements of both the body and the proboscis, and the latter has prob- 

 ably a respiratory as well as a muscular and a nutritive function, for the dendritic 

 outgrowths of its margins or ventral surface have much the appearance and structure 

 of gills and are situated in such a position that all water which enters the mouth 

 must first pass over them. Externally, in T. dendrorhynchus and T. branchio- 

 rhynchus, they are covered with ciliated epithelium; they contain spacious lumina 

 that communicate with the body-cavity by means of a longitudinal canal at their 

 base. The other parts of the proboscis are highly muscular, the bulk of the organ 

 consisting of a gelatinous substance that contains numerous bundles of longitudinal 

 muscle-fibres. In T. dendrorhynchus these are most numerous towards the ventral 

 surface, from which they proceed outwards, in transverse section, in somewhat ir- 

 regular bands that become gradually attenuated. Single transverse fibres run in the 

 opposite direction among the bundles and shorter fasciae also occur in the ventro- 

 dorsal axis between the bands. Immediately below the ectoderm on both ventral 

 and dorsal surface, there is a relatively broad horizontal muscle running across the 

 proboscis. 



