STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS. 
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the stomach, and that the greater portion of the small intestine is derived from part 
of the alimentary canal behind this, shows that Muller is mistaken in attempting to 
homologise the bursa entiana of Elasmobranchii, which is placed in front of the 
vitelline duct, with the coiled part of the small intestine of the above forms. The 
latter is either derived from an elongation of the very short portion of the intestine 
between the vitelline duct and the primitive 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,'* 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 Lepidosteus is, so far as we know, quite exceptional. This 
mesentery is shown in section in the embryo in Plate 25, fig. 53 ( v.mt .). 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 Lepidosteus 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 Muller 
(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 Muller pseudobranch). Muller’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 embryology 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 shown above, is never fully developed, and early undergoes a 
complete atrophy—a fact which is, on the whole, against Muller’s view; but what 
astonished us most in connexion with the gill in question is that we have been unable to 
find any trace of it even in the oldest larva whose head we have had (26 millims.), and 
at a period when the gills on the hinder arches have reached their full development. 
We imagined the gill in question to be the remnant of a gill fully formed in extinct 
Ganoid types, and therefore expected to find it better developed in the larva than in 
the adult. That the contrary is the fact appears to us fairly certain, although we 
cannot at present offer any explanation of it. 
* ‘ Comparative Embryology,’ vol. ii., p. 514. 
