NILS ROSEN, STUDIES ON THE PLECTOGNATHS. 



11 



The most primitive stage shows Lactophrys, in which only 

 the first pair of efferent branchial arteries enters tlie circuliis 

 cephalicus. The other three 

 pairs of these vessels join sepa- 

 rately the median aorta, which, 

 however, is very short (Fig. 

 8 a). The only specialization 

 here present is a shortening of 

 the median aorta. It repre- 

 sents a stage just a little ear- 

 her than that we find in Bali- 

 stes. RiDEWOOD found in B. 

 aculeatus the third and fourth 



efferent vessels to be U ni t ed Fig, 6. Bulbus arteriosus (b) and 



before reaching the median efferent branchial arteries (1-4) of 



f Balistes vctula [a) and Diodon hola- 



aorta (i^ig. 7 a), and the same can^/it/s (6). 



condition I found in B. vetula. The circulus cephab*cus is 

 small and the median aorta very short, as in Lactophrys. 



.^/^^^'\l There is a great gap 



XX. 



ZJI 



a 



W^ 



c 



between these some- 

 what primitive arrange- 

 ments and the condi- 

 tions found in the other 

 Plectognaths, that ha ve 

 been studied either by 

 RiDEWOOD or by my- 

 self. In all these forms 

 we meet with a very 

 large circulus cephalicus 

 that involves all the 

 efferent vessels. In Or- 

 thagoriscus (Fig. 7 c) the 

 third and fourth arte- 

 ries are united before 

 their entrance into the 

 circulus. In Diodon 

 (Fig. 8 b), Tetrodon 

 (Fig. 7 b) and Sphe^ 



Fig. 7. Circulus cephalicus of Balistes acu- 

 leatus (a), Tetrodon palembangensis (b) and Or- 



thagoriscus truncatus (c) (After Ridkwood.) 1—4, roides the fourtli artery 



first - fourth efferent branchial arteries (a. ^^S disappeared along 



branch. eff.); s, a. subclavia; a, aorta; cm, a. 

 coeliaconiesenterica. 



w4th the fourth gill 



