1907-] Records of tlic Indian Museu»i. 6q 



6. The outline of the sphincter is almost the same. 



7. The number and position of the mesenterial foramina are 



the same. 



Physical differences between the two forms. 



1. The column is much longer in the variety than in the typical 



form. 



2. Its wall is thinner during life. 



3 There is one cycle of tentacles and mesenteries less in the 

 variety than in the typical form. 



4. A larger number of mesenteries are usually rudimentary in 



the variety than in the typical form. 



5. The mesoderm of the mesenteries is thinner in transverse 



section in the variety than in the typical form and a 

 larger number of mesenteries are degenerate. 



6. There is no basal sphincter in the variety. 



In these lists only the resemblances and differences which 

 appear to be constant throughout life are noted. The following are 

 differences which are only apparent in full-grown individuals : — 



1. There are no muscle spaces in the sphincter of the typical 



form, while these spaces occur in small numbers in the 

 adult of the variety but are absent in its young. 



2. The adult of the variety is unable to withdraw its tentacles 



into its body, while the young of the same form and the 

 adult of the typical form can do so. 



3. The adult of the variety is unable to stand upright on its 



base, while the adult of the typical form and the young 

 of the variety can do so. 



4. The disk of the adult of the variety is broken up into 



lobes ; but this is not the case in the young of the 

 same form or the adult of the typical one. 



Biological differences between tlie two forms. 



The habits of the two forms are totally unlike. The typical 

 form lives in tidal waters, attached to solid objects; but it was also 

 found formerly in an isolated pond. The variety is apparently con- 

 fined to isolated ponds, the water of which sometimes contains as 

 little as 0'22 % of soluble solids ; the young live among grass-roots 

 and filamentous algae, or in the canals of Sponges, the adults buried 

 in the mud. Individuals of the typical form can live in water 

 of the same salinity as that of the isolated ponds in which the 

 variety occurs but are not now found in these ponds, from which 

 the solid objects to which they were formerly attached have 

 disappeared. The movements of the variety are more active 

 than those of the typical form, in accordance with the different 

 mode of life adopted. 



The most striking differences externally visible between the 

 two forms are the great relative length of the column and the 



