STURGEON-FISHES. 
1045 
leads into a canal which runs upwards, skirting ihe 
anterior margin of the great upper suspensorium Oto- 
mandibular, Jim) common to the whole apparatus of 
the palate and jaws, and opens (tig. 28G, spir) on the 
top of the head, a little behind the eye, at the outer 
margin of the osseous plate covering the squamosal 
part (squ) of the skull, and foreshadowing the squa- 
mosal bone. The upper opening is called the spiracle, 
and the canal, which as well as the former is entirely 
absent in the tishes we have hitherto considered, but 
which in all higher animals, man included, has its 
homologue in the external and internal auditory meatus 
and the tympanum, is here furnished in its lower 
(inner) part, just above the internal aperture, with a 
row of branchial lamella?, not respiratory, however, 
for they are supplied with arterial blood. The entire 
to the suboperculum and interoperculum. In the Sterlet 
these lamella? are for the most part free; in the com- 
mon Sturgeon they coalesce throughout the greater part 
of their external margin with the operculum, only their 
tops being free. In one of the Ganoids (Lepidosteus) 
Muller" has shown that the upper part of this row of 
gill-lamina 1 loses its functional importance as a respi- 
ratory organ, and it is thus fully homologous with the 
pseudobranchiae of the Teleosts. Behind the pharyngeal 
cavity the branchial cavity of the Sturgeon shows the 
same structure as that of the Teleosts, four gill-arches, 
each with two rows of branchial lamella? and short, 
scattered gill-rakers; and the fifth arch (fig. 284, hbr 5 
+ cbr 5 ), the lower pharyngeal of the Teleosts, furnished 
with one row of gill-rakers, is here, too, destitute of 
branchial lamella?, and has no gill-slit behind it, being 
P.5 
Fig. 284. Forepart of the endoskeleton in a Sturgeon. Partly after J. Muller and Parker. 
B, basal angle; 6’, notochord; cbr , — cbr., first — fifth ceratobranchial cartilages; cehy, ceratohyoid; cost l — cost 8 , first — eighth ribs; csp, spira- 
cular cartilage (according to Parker); cle, dental part of mandible; etl, lateral ethmoid; hbr ] — bbr-, first — fifth hypobranchial cartilages; hhy, 
hypohyoid; hm , hyomandibular cartilage, with the upper part covered by a parostotic disk; M, articular part of the mandible (hind part of 
the Meckelian cartilage); mx, maxillary; rn.pt, metapterygoid cartilage; N, neural arches; Na, nasal cavity; obsp, orbitosphenoid ; opt, foramen 
of the optic nerve; pi, palatine; pop, preoperculum (according to Parker); Ps, neural spines; psp, parasphenoid ; pt, pterygoid; qu, quadrate 
cartilage; 11, rostral cartilage; spl, symplecticum ; stliy, stylohyoid; tr, foramen of the nervus trigeminus', vg, foramen of the 
nervus vagus', vom , vomer. 
canal is analogous to a gill-slit, between the palato- 
mandibular and hyoid arches, the latter of which fur- 
nishes with its upper parts (the hyomandibular, fig. 
284, hm, and symplecticum, spl ) a suspensory apparatus 
to the former as well. The gill- slit next behind (between 
the hyoid arch and the first branchial arch proper) has 
its branchial lamella? set in a large, but single, arcuate 
row on the inside of the operculum, throughout the 
hind margin thereof, as well as on the inside of the 
plates situated below the. operculum and corresponding 
firmly coalescent. with the hind wall of the branchial 
cavity or anterior side of the scapular arch. 
In an adult state the mouth of the Sturgeons is 
entirely toothless; but their larva? have teeth, in form 
and distribution not unlike those of the Sharks, in their 
corneous structure resembling those of the Lampreys, 
- both in the upper and lower jaws and on the cerato- 
branchial bone ( cb )\ ) of the first branchial arch proper. 
These teeth are developed soon after the palato-mandi- 
bular arch, originally continuous and arcuate, has brok- 
Abh. Akad. Wiss. Berlin 1844, p. 133, taf. II, fig. 1 and taf. V, fig. 6. 
