STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS. 835 



portion of the intestine between the vitelline duct and the primi- 

 tive spiral valve, or more probably by the conversion of the 

 anterior part of the intestine, originally provided with a spiral 

 valve into a coiled small intestine not so provided. 



We have already called attention to the peculiar mesentery 

 present in the adult attaching the posterior straight part of the 

 intestine to the ventral wall of the body. This mesentery, which 

 together with the dorsal mesentery divides the hinder section of 

 the body-cavity into two lateral compartments is, we believe, a 

 persisting portion of the ventral mesentery which, as pointed out 

 by one of us 1 , is primitively present for the whole length of the 

 body-cavity. The persistence of such a large section of it as 

 that found in the adult Lcpidostens is, so far as we know, quite 

 exceptional. This mesentery is shewn in section in the embryo 

 in Plate 38, fig. 53 (I'.Jiit.}. The small vessel in it appears to be 

 the remnant of the subintestinal vein. 



THE GILL ON THE HYOID ARCH. 



It is well known that Lcpidostens is provided with a gill on 

 the hyoid arch, divided on each side into two parts. An excellent 

 figure of this gill is given by Miiller (No. 13, plate 5, fig. 6), who 

 holds from a consideration of the vascular supply that the two 

 parts of this gill represent respectively the hyoid gill and the 

 mandibular gill (called by Miiller pseudobranch). Miiller's views 

 on this subject have not usually been accepted, but it is the 

 fashion to regard the whole of the gill as the hyoid gill divided 

 into two parts. It appeared to us not improbable that embryo- 

 logy might throw some light on the history of this gill, and 

 accordingly we kept a look out in our embryos for traces of gills 

 on the hyoid and mandibular arches. The results we have arrived 

 at are purely negative, but are not the less surprising for this 

 fact. The hyomandibular cleft as shewn above, is never fully 

 developed, and early undergoes a complete atrophy a fact which 

 is, on the whole, against Miiller's view ; but what astonished us 

 most in connection with the gill in question is that we have been 



1 Comparative Embryology, Vol. II. p. 514 [the original edition]. 



