1907] Records of the Indian Museum. 55 



is similar, but the sphincter contracts more strongly. The space in 

 which the disk is to be contained is therefore less, and the tentacles 

 are forced to dispose themselves in a different manner. The outer 

 cycles draw together in such a way that their tips are in contact or 

 almost in contact , while the inner cycles bend downwards and enter 

 the mouth and stomodaeum. The difference between the two ways 

 in which space is found for the bestowal of the tentacles during con- 

 traction of the disk is strikingly illustrated in bisected specimens of 

 the two forms. In full-grown individuals of the new race, on the 

 other hand, the tentacles and the disk are not withdrawn entirely 

 into the column when the animal is irritated, but, after partial re- 

 traction of the disk and contraction of the tentacles, the sphincter 

 contracts below the disk and the mouth is closed, not always very 

 tightly. This difference is connected with a change in habits which 

 will be discussed later. 



Basal disk. — 



Not the least striking difference between the two forms is 

 that connected with the basal disk ; but as in other characters, 

 the difference in this respect is more marked in fully grown in- 

 dividuals than it is in the young. The base of the typical form 

 is strongly muscular, that of the variety much more feebly so ; but 

 that of young examples of the variety resembles, in its general 

 characters, except in the absence of a sphincter, that of the typical 

 form. In the typical form, the main axis of the base forms a right 

 angle with that of the column, and the edge dividing them is 

 sharply defined. It is possible, however, for the basal disk to be 

 extended beyond the column under certain conditions, as when 

 the animal is stationed in a cavit}^ the diameter of which is 

 greater, but not very much greater than that of its column. The 

 lower surface of the basal disk is always flat as a whole. In young 

 examples of the new race the lower surface of the basal disk 

 is also flat ; but the edges do not appear to be extensile. In well- 

 grown individuals of this form, however, the lower surface 

 of the basal disk is not flat, but either concave or convex in 

 accordance with external circumstances. In fact, it has to a great 

 extent lost its functions as an organ of adhesion, in accordance with 

 the change of habits already alluded to. In both forms of the 

 species, there is a pore in the centre of the basal disk, communicat- 

 ing on the one hand with the ccelenteron and on the other with 

 the exterior. 



In young examples of the new race there is a distinct folding 

 of the ectoderm in the neighbourhood of the basal disk, comparable 

 to that which occurs all over the column of the tj^pical form ; while 

 a trace of folding can even be discovered in the former position in the 

 adult of the isolated form. The arrangement of the inferior termi- 

 nation of the mesenteries is very variable in the new race, -in which 

 the two mesenteries of a pair often join together and end before 

 reaching the centre of the basal disk, while sometimes they do not 

 meet at all and run right to the edge of the central pore. 



