198 BULLETIN OF THE UNITED STATES FISH COMMISSION. 



and where ttie opercular efferent opening s is situated behind the audi- 

 tory capsule. At this point I would call attention to what I believe to 

 be an important embryological character which appears to distinguish 

 Lophobranchiate embryos from those of the normal types of fishes. It 

 is usual to find the gill-openings of embryo fishes more or less uncovered 

 when they first appear. That is, the opercular fold is often so short as 

 to scarcely cover more than the first cleft ; this is a very marked feature 

 in Clupeoids, such as the shad, but is less marked in all other types 

 which I have observed. In the Lophobrauch embryo the superficial 

 epiblastic layer, which roofs over the gill-chambers, is apparently never 

 broken through until late, and at no time do the clefts and arches come 

 to be completely exposed as in the young shad. The opercular opening 

 appears late as a mere spiracle, and not as in other forms is the opercle 

 developed from before backwards. The simple opercular plate of the 

 Lophobranchiate embryo probably originates as an outgrowth from 

 behind the hyomandibular bone from a tract of mesoblastic tissue, 

 which appears comparatively late, since the opercle is not yet developed 

 in the stage represented in our figure. Of course we cannot yet be sure 

 as to the value of this character until we know more of the development 

 of other forms. The reuiarkable manner in which the operculum of 

 Gamhusia is developed warns us to be cautious in putting an estimate 

 upon such features, for here a hollow membranous process from the 

 yelk-sack extends up over the opercula, a feature quite as singular as 

 that noted in the Lophobrauch. 



There is a sharp bend in the oesophagus oe, and a little way below 

 this bend the alimentary canal suddenly widens. Dorsally and about 

 on a level with the middle of the pectoral fin the spacious air-bladder 

 ab arises as a diverticulum from the intestine; its connection with the 

 intestine is closed very early. In front of it and at one side the liver 

 Iv is developed, but I have not been able to make out where it joins the 

 intestine, which, for well-known morphological reasons, it must do ; it is 

 therefore represented only in outline. 



IsTearly opposite the commencement of the dorsal 1 find a very singular 

 valve in the intestine at iv. Nothing comparable to this structure has 

 been observed in fish embryos as young as this except by myself in the 

 posterior portion of the intestine of the larval cod (Gadvs), but in that 

 form it is only a constriction, and does not completely shut off the ante- 

 rior portion of the alimentary canal from the posterior. Beyond this 

 valve the intestine of the young Hippocampus is continued as a pyriform 

 rectum ending in the vent v, around which the rudiment of the sphinc- 

 ter ani muscle is apparent, through which the ano-cloacal canal passes, 

 receiving dorsally a duct from the urinary vesicle or bladder al, into 

 which the segmental ducts w of each side empty their products. The 

 extent of the development of the segmental ducts and mesonephros or 

 kidney could not be made out from my mounted specimen; this can 

 only be done by the help of transverse sections. 



