34 ox THK GILL-RAKERS OF THE SPOONBILL STURGEON. [May 3, 



Doubtless these vai-iatioiis in foiiu aie coii-elated with clifterences 

 in the natni-e of the food in the diflei'ent genera. 



The most important features with I'egai-d to the gill-i-akers of 

 Poh/ochm may be summnrised as follows : — 



1. The gill-iakers are setiform structui-es, 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 matiix of a gill-rakei- contains numerous lacunae which 



are connected with one anothei- by means of canaliculi. 

 In the shaft portion it contains, in addition, a sei'ies of 

 anastomosing channels in which lie blood-capillaries and 

 some loose connective tissue. 



3. Structurally, a, gill-raker bears a, very close i-esemblance to 



osteo-dentine. 



4. The mucous membiane coveiing a branchial arch is prolonged 



over each gill-raker in the foi-m of a complete investing 

 coat. 



5. The necessaiyr movements of the gill-iakers appeal' to be 



brought about by means of the contraction of some muscle- 

 fibi-es Avhich are inseited on the outside of each gill-raker 

 and which pass downwaixls and inwards to be attached to 

 the branchial arch. 



6. The food of Polyodon consists of microscopical organisms 



and the gill-iakers serve as a straining-mechanism which 

 effectually bars the entiy of such particles into the gill- 

 cavities. 



EXl'LANATION OF PLATE II. 



1. A portion of tlie 2nd branchial arcli of Polyodon spatlnda sliovviiisi- the 



general arrangement of the gill-rakers. (J nat. bize.) 



2. A semi-diagrammatic section taken across the 2nd branchial arch (tlirougli 



the ceratobranchial cartilage), to show the position of the gill-rakers in rela- 

 tion to the septum. (* lai'ger 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 of a gill-raker by means of elastic fibres to 



the cartilage of the branchial arch. (X 5.) 



6. A portion of the broadest part of the shaft of a gill-raker which has been 



thinned Ijy 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 ai-e seen to anastomose with one another at frequent intervals. 

 (X 40.) 



7. Five typical lacuna^ from the shaft of a gill-raker ; thej' are seen to be in 



communication with one another bj' 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 20.) 



