THE CLUPEOID CRANIUM AND THE SAYIMBLADDER 449 



served in 10 per cent formalin. Sections of embryonic stages of 

 Stolephorus mitchilli were also available. 



The cranial structure in all these species is essentially the 

 same, but the skull of P. pseudoharengus is less specialized than 

 in the other species. Hence it may be used as a type form. All 

 descriptions in this paper refer to this species, unless otherwise 

 stated. 



GENERAL RELATIONS OF THE SWIMBLADDER AND THE PRE- 

 COELOMIC DIVERTICULUM 



The swimbladder, in the species investigated, has the form and 

 relations typical of the Clupeoids. It consists of a fusiform, 

 tubular organ of small caliber, which runs the length of the 

 dorsal part of the visceral cavity. Posteriorly it tapers rather 

 suddenly, and either ends blindly or opens to the exterior by 

 communicating with the anus. Anteriorly it usually tapers 

 slightly and terminates in a rounded end in the region opposite 

 the third or fourth vertebra; cephalad to this it sends up into 

 the base of the skull a bifurcated precoelomic diverticulum 

 which brings the swimbladder into direct relation with the 

 membranous labyrinth. The pneumatic duct is open in these 

 fishes; it arises near the middle of the swimbladder and opens 

 into the blind end of the V-shaped stomach sac. 



The only important variations from the typical form in the 

 species under consideration are in Stolephorus. In this genus a 

 constriction near the middle of the swimbladder divides it into an 

 anterior part which retains the typical tubular form and a 

 posterior part which expands into a thin-walled chamber. The 

 pneumatic duct springs from near the anterior end of this cham- 

 ber. Similar relations are found in Pellona (de Beaufort, '09) . 



Through the greater part of its extent the swimbladder is 

 covered on its ventral surface by peritoneum; dorsally, it is in 

 contact with the body wall and separated from the vertebral 

 column by the aorta and kidney; areolar tissue, in some places 

 containing a large amount of fat, connects it with the strong 

 aponeurosis which bridges across the intercostal spaces. 



