igiQ-J B. Prashad : Echiuroids from Chandipore. 401 



The longitudinal muscles form a continuous sheath and are 

 not divided into bundles. 



There are two pairs of segmental organs with their external 

 openings behind the level of the ventral hooks. The vesicle is an 

 elongated bag-like structure with the free closed end pointing 

 backwards, and has the mouth of the internal funnel drawn out 

 into very long spiral lobes. 



The anal vesicles are of a simple type, about half the length 

 of the body, and -each provided with two rows of ciliated funnels 

 on their anterior half. There is nothing special to note with re- 

 gard to the rest of the anatomy. 



The animal preserved in spirit after fixation in formalin is of 

 a pale yellow colour ; the proboscis, however, is very much lighter 

 in tint, being creamy white. 



Type specimen : — W ""P in the collection of the Zoological 

 Survey of India (Indian Museum). 



The species under consideration belongs to the group of 

 Thalassema treated of in my paper referred to above. The 

 general anatomy and form are very similar ; the most important 

 feature, however, in which it differs from the forms discussed in 

 that paper is the low grade of development of the proboscis. In 

 discussing the different grades of development of this structure in 

 the various species I stated, that in T. sahinum we have a form 

 which shows the origin of small processes from the ventral mar- 

 gins of the proboscis, that in the second species T. dendrorhynchus 

 the processes are much better developed even becoming dendritic 

 by division, while in T. hranchiorhynchus the processes are still 

 better developed forming regular gills. The condition in the pre- 

 sent form is even more primitive than in T. sahinum, for there is 

 only a proboscis of a rudimentary type without any processes. 

 T. microrhynchus in this respect represents probably the most 

 primitive member of this group of the genus Thalassema. 



A point of great biological interest arises from the occurrence 

 under similar conditions of two such diverse forms as T. hranchior- 

 hynchus and T. microrhynchus, which stand at two extremes as 

 to the development of the proboscis and the branchial processes. 

 Specimens of the two species have been collected from the same 

 locality, living under apparently similar biological conditions. 

 The lines of evolution of respiratory structures in the two forms, 

 however, are as divergent as possible. Whereas in T. hranchio- 

 rhynchus the respiratory surface has been very greatly increased by 

 the/development of a large proboscis and very long, branched bran- 

 chial processes, in the other {T. microrhynchus) the proboscis is 

 quite rudimentary. Similar cases of adaptive characters of exactly 

 opposite type developed by two species living in similar biological 

 surroundings are not unknown elsewhere, and reference may be 

 made to the various examples amongst sponges cited by Annan- 

 dale.^ In all these cases the two species have developed or at 



1 Joiirn. As. Soc. Bengal (n. s.), IX, p. 75 (1913), Mem. Ind. Mtis., V, p. 

 54 (1915) and Mem. As. Soc. Bengal. VI, pp. 196-197 (1918). 



