UNUSUAL FORM Or UEMIPHOLTS. 143 



their free surface slants downwards and outwards aud away from 

 the axial line. They clearly caunot be approximated to chew. 



The use of the teeth and other structures on the end of the 

 jaw-angle and of the tooth-papillae is to filter the water which is 

 constantly passing into the stomach, to keep out particles which 

 might be too large and also living intruders. A movement of the 

 jaw-angles towards each other and the central axis of the mouth, 

 sufficient to bring the teeth in forcible apposition, does not seem 

 possible. But in some species there is a slight enlargement pos- 

 sible of the so-called mouth ; and the series of teeth of each jaw- 

 plate may be more or less distant from their fellows. Contrac- 

 tion, however, beyond a certain limit would cause crushing of the 

 delicately spinuled teeth of such forms as Ophioscolex. The en- 

 largement of the mouth and consequent separation of the jaws is 

 common in those genera the species of which can move the arms 

 upwards ; but this appears usually to be a convulsive action pre- 

 ceding death, a rigor mortis determines the persistence of the 

 condition, and post mortem rigidity permanently widens the space 

 between the jaw-angles in many forms. Nevertheless there are 

 species which have the circle of side mouth-shields discontinuous, 

 whose arms are capable of a slight upward and aboral movement, 

 and whose mouth-canal can be widened and narrowed during life. 

 Mr. W. Percy Sladen, F.L.S., is now investigating the disk and 

 arm attachments and insertions of some hitherto unnoticed mus- 

 cular slips which appear to explain this movement. 



I have named this species after its discoverer, Hemipliolis 

 Wallicliii. 



EXPLANATION OF PLATE VI. 

 Fig. 1. Hemipholis Wallichii, Duncan, about nat. size. 



2. The under part of the disk and arms, magnified. 



3. The upper part of the disk and arms, magnified. 



4. Diagrammatic section from above downwards of the disk, showing 



the projecting spine on the side mouth-shield. 

 5. Arm-spine, magnified. 



