THE DOGFISH 15 



In the heart make out ventrally a thick-walled sac, the 

 ventricle and, extending forward from this to the anterior wall 

 of the pericardium, a muscular tube, the truncus arteriosus. 

 Dorsal to the ventricle and visible on either side of it from below 

 is a larger and thinner-walled sac, the auricle, behind which, 

 and connecting it to the septum transversum, is a thin-walled 

 cavity, the sinus venosus. 



At the lateral angles of the sinus venosus trace the connections 

 of the lateral abdominal veins, and in the middle line the hepatic 

 sinus already mentioned. Other veins, to be added later, enter 

 the sinus. 



Insert heart, pericardium, hepatic veins, lateral abdominal 

 veins, etc., in the sketches of the circulation already made. 



Insert one point of the scissors in a gill-slit, the scissors point- 

 ing ventrally and toward the middle line. Carry the incision 

 nearly to the middle and repeat the operation with the other 

 slits of the same side. This will open the gill-slits, which are 

 now seen to be separated by partitions, the gill-septa. In the 

 middle cleft see the gill-filaments or, better, folds, on anterior and 

 posterior walls. Does the septum project beyond the gill-folds? 

 Are there any folds on the dorsal or ventral walls? If we 

 speak of the gill-folds borne on a septum as constituting a 

 gill or branchia, then those on either side of a septum would 

 constitute a demibranch, while one gill-cleft will include demi- 

 branchs belonging to two gills.* Do all the gill-clefts bear two 

 demibranchs? Split a bit of the septum near the dorsal and 

 ventral margin of the gills and see, at the level of the ends of the 

 gill-folds, the small extrabranchial cartilages (p. 8). 



Sketch the gills as laid open. 



Cut through the skin in the median line from the pericardia! 

 cavity to the symphysis of the lower jaw. In front, just behind 

 the skeleton of the jaw (Meckel's cartilage) and the hyoid arch, 

 find a large glandular mass, the thymus gland. 



Now cut deeper in the middle line, beginning behind and 

 following the truncus arteriosus from the pericardium through 

 the muscles of the throat. This portion is known as the ventral 

 aorta, or aorta ascendens. Notice the vessels arising from it 



* Frequently the demibranchs of a cleft are regarded as forming a gill, 

 but the view here adopted renders comparisons more easy with teleosts and 

 amphibia. 



