4 ARKIV FOR ZOOLOGI. BAND 7. N:0 2o. 



auricle. It extends forwards a good distance beyond the 

 anterior apex of the auricle, and when seen from the ventral 

 surface only its posterior left part is covered by the auricle. 

 The axis of the ventricle in the position it has in the peri- 

 cardial cavity points upwards and forwards. The communi- 

 cation between the auricle and the ventricle is almost in the 

 horizontal plan from the left to the right. There are two 

 vertical auriculo-ventricular valves, one anterior and one 

 posterior. The ventricle continues in the short conus arierio- 

 sus, the entrance to which is guarded by one pair of semi- 

 lunar valves. On the conus the enlarged muscular bulbus 

 arter iosus follows. 



m 



Fig. 2. Heart of Diodon holacanthus L. (young specimen). a, ventral 

 view ; b, dorsal view ; c, from the right side ; d, transverse section, through 

 the auriculo-ventricular opening. Reference letters as in Fig. 1. 



In Diodon we meet with a heart of a shape rather diffe- 

 rent from that described in Spheroides. In a young specimen 

 of Z). holacanthus the auricle has an irregular rounded form, 

 the ventral suface v/ith a broad longitudinal groove for the 

 vertricle (Fig. 2). The ventricle is quite ventral in position 

 to the auricle, and the communication between the two parts 

 of the heart is in the vertical plan. The ventricle is oblong 

 and keeled on its ventral surface. In a fullgrown specimen 

 of D. hystrix I found the auricle to be very irregular, on the 

 right side with a large triangulär flap descending. This flap 

 forms an impression of the same form on the right side of 

 the ventricle. The ventricle was in this species strongly com- 

 pressed, but not keeled on the ventral surface. In both 



