CHONDROCRANIUM OF SYNGNATHUS FUSCUS 453 



of respiration, since Syngnathus at this stage is a free-swimming 

 larva. An osseous lamella lies external to and separate from 

 the ventral portion of the hyomandibula, enclosing a blood- 

 vessel between it and the perichondrium (fig. 13). 



As in the 8-mm. stage, the stylohyal element is interpolated 

 beween the dorsal end of the ceratohyal and the hyomandibula 

 (fig. 14). The ceratohyal, which in the same stage (when the 

 embryo is yet enclosed in the brood pouch of the parent) extends 

 anteroposteriorly in an almost horizontal plane with the posterior 

 end in the more ventral position, has now changed in position, 

 so that it lies almost vertical (fig. 10). The dorsal end of each 

 ceratohyal articulates with the stylohyal of that side, while the 

 ventral end, formerly parallel with the hypohyal element, now 

 lies ventroposterior to it. 



The hypohyal element has also undergone a change in position. 

 Instead of lying in a horizontal plane parallel to the copula 

 communis, it now occupies a vertical position, articulating on 

 its mesiodorsal surface with the copula communis (fig. 14). 

 The change in the positional relationships of these elements is 

 due to the functional activity of these as the supporting cartilages 

 of the oral cavity, which by its changes in shape draws water 

 into the mouth and expels it over the gills. Since the gills were 

 not developed in the 8-mm. stage, these parts had not shifted. 

 The conditions of these elements in the 12-mm. stage of Syn- 

 gnathus are more nearly like those of the 6.6-mm. Gasterosteus, 

 which indicates an earlier functional activity of these parts 

 in the latter, correlated with the longer protected period of 

 Syngnathus. 



The copula communis of the 12-mm. stage has retained the 

 same relative length as before, but is more widely separated from 

 the ventral surface of the cranium, because of the increase in 

 the size of the oral cavity (figs. 10, 14). Posteriorly it extends 

 beyond the second branchial arch after having been displaced 

 ventrally between the hypohyal elements due to the shift in 

 their position. Its posterior end is connected by a cord of 

 procartilage cells with the independent basibranchial cartilages 

 between the third and fourth branchial arches. 



