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Show 34 OX THE GILL-RAKERS OF THE SPOONBILL STURGEON. [May 3, Doubtless these variations in form are correlated with differences in the nature of the food in the different genera. The most important features with regard to the gill-rakers of Polyodon may be summarised as follows- 1. The gill-rakers are setiform structures, and each consists of a basal portion, which is attached by means of elastic fibres to the branchial arch, and a long, free, shaft portion which forms its principal part. 2. The matrix of a gill-raker contains numerous lacunae which are connected with one another by means of canaliculi. In the shaft portion it contains, in addition, a series of anastomosing channels in which lie blood-capillaries and some loose connective tissue. 3. Structurally, a gill-raker bears a very close resemblance to osteo-dentine. 4. The mucous membrane covering a branchial arch is prolonged over each gill-raker in the form of a complete investing coat. 5. The necessary movements of the gill-rakers appear to be brought about by means of the contraction of some muscle-fibres which are inserted on the outside of each gill-raker and which pass downwards and inwards to be attached to the branchial arch. 6. The food of Polyodon consists of microscopical organisms and the gill-rakers serve as a straining-mechanism which effectually bars the entry of such particles into the gill-cavities. E X P L AN A T IO N OF PLATE II. Fie. 1. A portion of the 2nd branchial arch of Polyodou spathnta showing the general arrangement of the gill-rakers. (| nat. size.) 2. A semi-diagrammatic section taken across the 2nd branchial arch (through the ceratobranchial cartilage), to show the position of the gill-rakers in relation to the septum. (^ larger than nat. size.) 3. A single gill-raker. (Nat. size.) 4. A section taken through the lower part of a gill-raker which has been decalcified and afterwards stained with Kleinenberg's hsematoxylin. The mucous membrane of the branchial arch is prolonged over the surface of the gill- raker. The matrix of the latter shows indications of lamination and numerous lacuna? are seen scattered through it. (X 30.) 5. Figure showing the attachment ot a gill-raker by means of elastic fibres to the cartilage of the branchial arch. (X 5.) (S. A portion of the broadest part of the shaft of a gill-raker which has been thinned by rubbing down upon a. fine hone, the preparation afterwards mounted in balsam. The matrix is seen to contain large numbers of lacuna; mostly with their axes pointing in one direction. The vascular channels are seen to anastomose with one another at frequent intervals. (X 40.) 7. Five typical lacuna; from the shaft of a gill-raker; they are seen to be in communication with one another by means of their canaliculi. From a preparation made in the same way as the one represented in fig. 6. (X 370.) 8. View of the extremity of the shaft of a gill-raker mounted whole in glycerine. This figure shows the termination of the vascular channels in the tip of the same. (X '2 0 .) |