8 



ARKIV FÖR ZOOLOGI. BAND 10. N:0 8. 



things. The haemal canal is made uj) of the ventral arches. 

 Haemal processes as well as ribs are absent. It is at least 

 impossible to distinguish such portions. A study of the 

 arrangement of the body muscles makes the problem clear. 

 The horizontal septum which separates the dorsal body muscles 

 from the ventral ones inserts not, as in other fishes, to ahoiit 

 the ccntre of the vertebrae but much more ventrally to the ven- 

 tral arches not far from their tips. The dorsal body muscles, 

 which are very well developed, have displaced the septum and 

 the ventral arches ventrally. The ventral arches of both sides 

 point ventrally and have been, so to say, pressed towards the 

 median line, so that their tij)s join each other. In the haemal 



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Fig. A. — Mola, young specimen, 1,5 mm. in length. Diagram of a part 

 of the vertebral columin (prsecaudal region). 



canal the aorta runs. We have without doubt to find the 

 explanation of all these peculiarities in the power of inflating 

 the body which the Tetrodontids possess. The haemal canal 

 of the caudal region seems to be formed in the same mode as in 

 the praecaudal one. That ribs do not take part in the formation 

 of the caudal haemal canal is not stränge, if we accept Schau- 

 insland's theory of the nature of the ribs. According to 

 him the ribs are portions of the ventral arches, which have 

 been separated. This view explains all the different opinions 

 that have been put forth about the haemal canal. 



In the young specimens of Spheroides described above the 

 chorda shows irregular vacuoles. A disinct chorda-funiculus 

 formed by the compression of the central vacuoles is present 

 in this stage. 



