360 PHILOSOPHICAL TRANSACTIONS. [aNNO 173Q. 



The upper part of the external convex surface, a, fig. 8, has a very sharp 

 edge, in which the first series of fibres begins from a point ; which fibres 

 gradually dilating, and running lengthwise, end about the middle part of it, 

 which makes a right angle with the upper part. In this part the fibres being 

 elevated, run crosswise, e, fig. 8. The lower part is thicker than the upper, 

 but softer and less compact. In this part the fibres are raised up and rough, 

 first curved, then straight, and, like the others, run lengthwise to the lower 

 edge of this part, which is strongly fastened to the head by various ligaments 

 c, D. 



The concave or inner part of these hemicrania, which contains the softer 

 parts of the head, is very smooth ; but almost in the middle has a very small 

 and tender eminence or process, in shape much like Dr. Rau's process in the 

 organ of hearing, fixed at one end, and loose at the other, running almost the 

 whole width, and doubtless destined for supporting some of the inner parts of 

 the head. See a, fig. g. 



These two hemicrania, connected together by strong ligaments, and as it 

 were by a small hinge, by means of which they can dilate without separating, 

 besides their defending the soft head from external injuries, are the instruments 

 with which the animal gets its food. For whatever way it turns its head, the 

 raised and rough fibres, running either lengthwise or crosswise, always rub off 

 some of the wood. 



These hemicrania being carefully removed, the contained parts, fig. 1 1 , are 

 laid open to view ; but they are so soft, and of so wonderful a structure, that 

 the eye, though armed with a microscope, can neither discern their true make 

 nor use. First indeed there appears a membrane, enveloping the whole head ; 

 in the middle and anterior part, which is not covered by the said hemicrania, it 

 appears as if raised by a tubercle c, and in that place it is of a red colour ; but 

 the lower ligamentous edge firmly adheres, both to the small process, and to 

 the lower edge of the hemicranium. 



This membrane carefully separated and removed, a, a, fig. 12, in the middle 

 of the subjacent pulp is a small pear-like body, perfectly pellucid, somewhat 

 protuberant above the other parts, which made the tubercle in the membrane. 

 It is much harder than the other contained parts of the head and body ; so that 

 it will bear cutting with the scalpel. It is of a red colour, as perfectly pellucid 

 as a drop of water; of the shape of a pear, from a larger basis terminating in a 

 point. It cannot be better resembled to any thing, than to the crystalline lens 

 of the eye : yet in spirit of wine it preserved its transparency, but its bulk was 

 diminished, as b, fig. 12. 



At the sides, where the lower edges of the hemicrania do not touch each 



