28 MR. A. D. IMMS ON THE GILL-RAKERS [May 3, 



In a gill-raker which has been decalcified in a 10 per cent, 

 solution of acetic acid, and afterwai'ds stained with Kleinenberg's 

 hsematoxylin and cut into longitudinal sections with the microtome, 

 some additional features may be observed (PI. II. fig. 4). The 

 ground-substance exhibits indications of being stratified and its 

 layers show varied capabilities for absorbing staining-reagents. 

 Traversing it are numerous canals, which are lined internally by a 

 definite membrane and contain one or more blood-vessels and some 

 loose connective tissue. In the region of the shaft the lacunae have 

 protoplasmic contents which are i-eadily stained, together with one 

 or more deeply staining bodies which are apparently nuclei. Th(Jse 

 lacunae which are situated neai'est to the blood- channels can be 

 distinctly seen to be in communication with them by means of 

 their canaliculi. In the basal part of the gill-raker many of the 

 lacunae are shrunken in their outlines and are veiy poor in 

 stainable contents. The lower part of the central cavity, which 

 is represented at c.c, appears to be nearly empty, containing 

 only some nucleated tissue in which no definite cells are to be 

 distinguished. The elastic fibres of the ligament, already men- 

 tioned, penetrate deeply into the substance of the base in much 

 the same manner as the perforating fibres of Sharpey, which are 

 composed partly of bundles of elastic fibres, pierce the circum- 

 ferential lamellae in bone. In fig. 4 (e./!), where these fibres are 

 seen in section, they appear as variously shaped dots according to 

 the angle through which they have been cut, and they are very 

 evenly distributed through the peripheral ground-substance. The 

 principal blood-vessel is seen to enter the gill-raker about the 

 point of junction of the shaft with the basal part. The vessel 

 then breaks up into several branches, which penetrate the ground- 

 substance and reach the canals traversing it. The mucous mem- 

 brane of the branchial arch («^. in figs. 4 & 5), accompanied by 

 capillaries, is prolonged upwards as a complete and continuous 

 investment to the outer surface of the gill- raker. In a decalcified 

 gill-raker the presence of a membrane covering it is easy to make 

 out without cutting sections, as it can be sti-ipped off by using 

 a fine needle under a dissecting microscope. 



The only fish which possesses gill-rakers at all comparable with 

 those of Polyodon is Getorhinus maximus. In this species the 

 gill-rakers are of the same general form and, in proportion to the 

 much greater size of the animal, they are correspondingly larger 

 and stouter. The investigations of Hannover * and Turner t have 

 shown, on histological grounds, that there is good reason for 

 believing them to be very greatly elongated teeth. In common 

 with those of Polyodon, 'they consist at their bases of a matrix 

 permeated with anastomosing canals containing blood-vessels ; in 

 the shaft they difier in that they contain but a single canal which 

 runs straight up to the tip. There are no lacunae, but the matrix 



* Kong. Danske Videiisk.-Selskabs Skrifter, 1868, p. 483. A resume is given iu 

 French in die Ann. Sci. Nat., Zool. t.ix. 1868, p. 373. 

 t Journ. Anat. & Phys. xiv. 1879, pp. 273-286, pi. xii. . . ' . '- 



