2sILS ROSÉN, STUDIES ON THE PLECTOGNATHS. 9 



roides is to be regarded as an intermediate stage in the tor- 

 sion of the heart-tube, or as a secondary retorsion, cannot 

 be decided before the developmcnt of the heart has been 

 studied. Neither in the mode in which the blood vessels enter 

 or arrise from the heart in this fish, nor in other struetures 

 of the heart or in the shape of the pericardial cavity, have 

 I been able to find anything that throws light upon this 

 singular condition. Bnt anticipating the results about the 

 phylogeny of the bloodvessels in the Plectognaths to which 

 I have come in this paper I think the situation of the 

 auricle in Spheroides is a secondary one. 



2. Arteries. 



Truncus arteriosus, branchial arteries and circidus cepha- 

 licus. On the conus arteriosus follows the truncus, the caudal 

 part of which in all the Plectognaths I have examined forms 

 a distinct and muscular bulbus. In Balistes it is of a trian- 

 gulär shape (Fig. 6 a, 6), in Diodon (Fig. 6 b, 6), Spheroides 

 (Fig. 1 a, h) and Lactophrys more cylindrical or conic. The 

 cranial part of the truncus is long and slender in Balistes, 

 shorter in Diodon and Spheroides, in which latter form it is 

 exteriorly hardly distinct from the bulbus. From the truncus 

 the afferent branchial arteries arise. Their number varies 

 according to the number of gills : 4 in Balistes (Fig. 6 a), 3 

 each in Spheroides (Fig. 1 a) and Diodon (Fig. 6 b). In the 

 former genus the third and fourth arteries are united at their 

 base. Orthagoriscus and Lactophrys I have not examined on 

 this point. The circulus cephalicus and the relation of the 

 efferent branchial arteries to it show amongst the Plecto- 

 gnaths great differences. Ridewood has made very interesting 

 researches on the arrangement of these vessels in a number 

 of Teleosts. The most primitive condition is, without doubt, 

 a very small circulus cephalicus which does not involve more 

 than the first efferent artery of each side. In this simple stage 

 all the efferent arteries enter the circulus or the aorta separately. 

 Such primitive conditions Ridewood found in Engraidis and 

 Clupea, and from this he thinks that the specialization has 

 proceeded on two lines: »firstly, by the circulus cephalicus 

 involving the second, and låter the third and fourth efferent 



