456 THE DOGFISH 



PRACTICAL DIRECTIONS. 



Dogfishes arc iDcst preserved in 4 per cent, formaline, whieh has the 

 additional merit over spirit of not coagulating the blood, so that the 

 vessels can be injected in preserved specimens. They can be obtained, 

 fresh or ready preserved, from any Marine Biological Station. 



A. External characters : see pp. 415—418. Sketch from the side. 

 Isolate .some of the dermal teeth by boiling a small piece of skin in 



caustic potash, and make out the bon)' basal plate, and the spine com- 

 posed of dentine tipped with enamel. Sketch. 



B. Skeleton. (If J'ou are working on a fresh fish, and wish to 

 dissect the soft parts before preservation, the examination of the 

 skeleton may be postponed until later. ) 



It is advisable to have one .skeleton prepared entire, and one in 

 which the parts have been disarticulated. Obtain a common butcher's or 

 cook's pointed knife (a strong pocket-knife will do) for cutting through 

 the rough skin and for the coarser work of preparation. Prepare as 

 directed on p. 53, and when the greater part of the muscles has been 

 removed, disarticulate the .skull from the vertebral column, leaving the 

 branchial apparatus attached to it, and also remo\'e the paired fins and 

 their arches. Disarticulate the hyomandibular cartilage from the 

 cranium so as to separate the visceral arches, including the jaws 

 (compare Fig. 103) : these should then be thoroughly cleaned without 

 further immersion in hot water, as the cartilages of which they are com- 

 posed come apart very easily. The other parts may be dipped into 

 hot water for a few seconds from time to time, but care should be taken 

 that the more delicate elements do not thereby become separated. It is 

 useful to prepare a second cranium as well as a few trunk- and caudal 

 vertebra;, which should be bisected vertically into right and left halves. 

 When prepared, the skeleton should be kept in weak spirit or forma- 

 line, and not allowed to dry, or the cartilages will of course shrink, 

 unless the following method is resorted to : — 



Thoroughly clean a skeleton, or typical parts of it {e.ff. .skull, limb- 

 skeleton, and a few trunk- and caudal vertebrre), and then transfer from 

 weak into strong methylated spirit for a day or so, and afterwards into 

 absolute alcohol for a few hours. Place in a vessel filled with turpen- 

 tine for another day, and then transfer into melted paraffin in the water- 

 bath until the parts are thoroughly permeated, after which they should 



