2 ARKIV FÖR ZOOLOGI. BAND 10. NIO 8. 



possess. Brigde has described the mesial fins. In order to 

 give an account of the skeleton of the Plectognaths I have 

 examined adult specimens of Mola, Lactophrys, Balistes and 

 Spheroides and studied microscopical sections of young spe- 

 cimens of Mola and Spheroides. Thongh these studies are not 

 complete enough to give a quite characteristic description 

 they shall nevertheless be published here. I must point out, 

 however, that it is very difficult to get larvae of these fishes 

 representing a series of different stages of development. What 

 remains to complete our knowledge must be left to future 

 writers, who perhaps may get a chance to study larvae of 

 other stages than I have at my disposal. 



A. The vertebral column. 



Molidae. 



Several writers have given short descripticns of the ver- 

 tebrae. Lutken and Steenstrup state the number of the 

 vertebrae to be 16, the eight cranial of which have only dor- 

 sal arches, the others, except the last one also ventral ones. 

 The dorsal arches of the three first vertebrae do not touch 

 each other in the median line. Ribs are absent. Nothing 

 has hitherto been written about the development. Some re- 

 marks thereon will be made here. 



In a larva of Ilola measuring 1,5 mm. in length almost 

 the whole chorda is vacuolized. Only at the periphery is 

 this not the case. Cells of an epitheliomorfous structure are 

 to be found there, forming, except in a few points, a simple 

 layer. The vacuoles are all of about the same size (Pl. 1, 

 Fig. 1). A chorda-sheath is developed. In the sclerotome- 

 tissue surrounding the chorda ossification has already taken 

 place, which will best be seen in horizontal sections. The 

 metameres are bounded cranially and caudally by the myo- 

 septa. In the cranial parts of the metameres intersegmental 

 bloodvessels run. In each metamere two dorsal arches are 

 developed, one cranial and one caudal (Fig. A). The former 

 is very small, the latter much larger. They seem both to be 

 formed by bone-tissue; but, as it is ver}^ difficult to stain 

 the bone and cartilage of Mola, it is possible that the pre- 



