472 THE DOGFISH CHAP. 



its contents is constantly going on. By the time the young 

 fish is ready to be hatched or born, the greater part of the 

 yolk-sac has been drawn into the coelome, a mere remnant 

 of it still dangling from the ventral surface of the body. 



PRACTICAL DIRECTIONS. 



Dogfishes are best preserved in 5 pe'r 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 bony basal plate, and the spine composed of dentine 

 tipped with enamel. Sketch. 



B. Skeleton, (If you are working on a fresh fish, and wish to 

 dissect the soft parts before preservation, the examination of the 

 skeleton may be posponed 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, 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 

 trunk- and caudal vertebrae, which should be bisected vertically into 

 right and left halves, When prepared, the skeleton should be kept in 



