52 N. Annandai.E: The Fauna of Brackish Ponds. [Vol.. I, 



variety, however, the sphincter region can be distinctly recognized 

 externally, forming a somewhat corrugated and rather opaque 

 band beneath the disk, and measuring about one-twelfth of the 

 whole column in length. It is well shown in fig. 5, pi. iii. 



The basal sphincter is formed b}" a few comparatively deep folds 

 in the circular muscle at the base of the column round the periphery 

 of the basal disk. I can find no trace of it in the variety. 



Longitudinal muscle fibres can occasionally be detected in the 

 mesoderm of the column in the typical form ; in the variet}'^ they are 

 fairly abundant in the spindle-shaped swellings of the mesoderm 

 referred to in a preceding paragraph. 



In both forms of the species, the basilar muscles of the mesen- 

 teries are well developed, surrounding outgrowths of the mesoderm 

 at the base of these organs and having a dendritic outline in 

 transverse section. ks a rule they are developed almost equally- 

 on the two sides of the mesentery ; but their exact outline varies 

 greatl}^ even in different mesenteries of the same individual. The 

 basilo-retr actor muscles are on the other hand somewhat feebly 

 developed, accompanying a relatively slight folding of the meso- 

 derm often almost indistinguishable. They, too, are very variable. 

 The retractor muscles are stout and somewhat short in trans- 

 verse section in both forms In the variety it is possible to 

 distinguish these belonging to the directive mesenteries from the 

 others by their shape as well as by their position and orientation. 

 In transverse section all have a reniform outline but those of 

 the directive mesenteries are shorter and more nearly circular. 

 In the typical form of the species this characteristic is not so 

 marked as in the variety, but in the latter there is more space for 

 the muscles to retain their natural outline than there is in the 

 former. The retractor muscles in the variety become gradually 

 more slender near the base of the column, and practically disappear 

 before the base is reached. In the typical form, however, they 

 extend along the basal disk almost to its centre, and play an 

 important part in the muscularity of that structure. 



The radial muscles of the disk and tentacles are at first sight 

 difiicult to detect, owing to the fact that they form a relatively 

 narrow band in transverse section. In .suitable longitudinal 

 sections of the tentacles, however, they appear to be powerful and 

 are easily distinguished. 



Tentacles and disk. — 



The arrangement of the tentacles is closely similar in the two 

 forms, but the variet}^ generall}^ has one cycle fewer than the typical 

 form, full-grown individuals of both being examined. Stoliczka said 

 that he could distinguish the six primary tentacles from the others 

 by their shape ; this I have been unable to do, but, at any rate 

 in young individuals , their position surrounding the mouth is quite 

 distinct and the}^ are separated from the other cycles. Typically 

 each cycle, commencing from the primary- cycle and going out- 

 wards, has twice as many tentacles as the one immediatel}' within 



