472 THE DOGFISH CHAP. 



PRACTICAL DIRECTIONS 



Dogfishes are best preserved in 5 per cent, formaline, 

 which, unlike spirit, does not coagulate 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. 431-434. Sketch from 

 the side. 



Examine a small piece of the skin under the low power 

 with reflected light and note the form and arrangement of 

 the dermal teeth. Isolate some of these by boiling a small 

 piece of skin in caustic potash (p. 359), and make out the 

 bone-like basal plate, and the spine composed of dentine 

 tipped with enamel. Sketch. 



B. Skeleton. 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 pre- 

 paration. Prepare as directed on p. 53, dipping into hot 

 water occasionally, or macerating in 2 per cent, nitric 

 acid for a day or two. 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 remove the paired fins and their 

 arches. Disarticulate the hyomandibular cartilage from 

 the cranium so as to separate the visceral arches, includ- 

 ing the jaws (compare Fig. 112) : these should then be 

 thoroughly cleaned without further immersion in hot water, 

 as the cartilages of which they are composed 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 additional trunk- and caudal vertebra?, which should 

 be bisected vertically into right and left halves. When 

 prepared, the skeleton should be kept in weak spirit, and 



