112 ALLEN 



Liike Aci'^enser and Sca^hirhyncJms the ventricle of the heart 

 of Polyodon is conspicuous for the amount of lymphoid tissue 

 surrounding it. There is always one large lymphatic gland 

 encircling the anterior part of the ventricle ; another smaller 

 one is situated behind it on the lower side of the ventricle, in- 

 closing the greater portion of the posterior end of the ventricle. 

 It is very singular in Polyodon that the coronary artery^ which 

 supplies these glands as well as the heart, comes from the fourth 

 right efferent branchial artery and approaches the heart from 

 the rear. When the ventral side of the sinus venosus is reached 

 it trifurcates. Two of these branches pass cephalad between the 

 auricle and the ventricle, supplying each ; while the third follows 

 along the left and ventral border of the ventricle, and gives off 

 branches that traverse between the surface of the ventricle and 

 these glands, supplying each. Corresponding venous branches 

 arise from the anterior and left ventral side of the ventricle, 

 which likewise pass between these glands and the ventricle 

 to terminate in two longitudinal coronary veins that travel 

 caudad between the auricle and ventricle. At the posterior 

 apex of the heart these two veins anastomose, forming the 

 comynon coronary vein, which discharges itself into the sinus 

 venosus from below. For the most part the blood-vascular 

 network lies between these glands and the ventricle. In addi- 

 tion to the vascular network described above there is a coarser 

 and more superficial network, decidedly lymphatic in the char- 

 acter of its meshes, which encompasses these glands. This is 

 undoubtedly a lymphatic network, but so far as could be deter- 

 mined it was not collected by any definite lymphatic canal. 

 Consequently it must, as Robin found in Toi'^cdo {of. cit., pp. 

 15 and 16), reach the veins through the oesophagus plexus. 

 One finds upon sectioning this lymphoid tissue surrounding the 

 heart that it somewhat resembles the thymus. It consists of a 

 connective tissue framework which is completely filled with 

 leucocytes. In addition to the numerous blood vessels that 

 traverse it, many lymphatic spaces or sinuses were found 

 which were filled with leucocytes. 



