422 
MR. W. K. PARKER ON THE SKELETON 
becoming absorbed during metamorphosis, whilst two new pairs appear, homologous 
with the “ adrostral cartilages ” and “ nasal valves ” of the Elasmobranchs. 
I have already described the cartilages that are attached to the yreat ring, 
doubting whether they belong to the superficial or the deep category. 
There remains, now, the great annular cartilage to be described (Plate 18, figs. 1-2, 
l.l h), and the lesser pieces in the disk ( l.l 2 ~ 4 .). There can be no difficulty in identifying 
this huge, single, complete ring with the lesser, double, incomplete ring of the Tadpole, 
nor in determining that they are both distal superficial cartilages of the post-oral or 
mandibular region. I hare traced the development and decadence of this suctorial 
structure through all its stages, up and down, in the Anura (“ Skull of Batrachia,” 
Parts I.—III.), but in this type I find no traces of it in the largest Arnnnoccete of P. 
planeri and P. jluviatilis. But in a very minute young of P. marinus, 4 inches long, 
(Plate 10, fig. 6, Id 1 .)* with the cranial elements still distinct, the chondrilocation is 
equally perfect all round, and the thickness of the ring equal. At that stage, if it had 
been composed of two pieces, even for a day or two, some signs of division would have 
been apparent. The actual form in the larger young (one-third grown) of the same 
species (Plate 18, figs. 1, 2, l.l. 1 ) is a semi-ellipse, with rounded corners at the trun¬ 
cated part; the narrow end is in front. 
It has a thick lower edge (fig. 2) and thence is scooped upwards; the fore part 
(fig. 1) is much higher than the hinder; the narrow end projects much less in front of 
the large upper labial than it does beyond the cornu trabeculte (Plate 18, figs. 1, 3, 
l.l 1 ., u.l\, c.tr.). In the side view (Plate 18, fig. 1) the larger teeth, round the opening 
of the circular mouth, are shown in situ; in the dissected skull in the under view (fig. 2), 
the head of the tongue (lingual or basi-hyal cartilage) is seen with its teeth in the 
hinder part of the opening. Also in the undissected head, when seen from below 
(Plate 8, fig. 10), the same part is seen as a squarish mass, largely occluding the 
aperture ; the teeth of the disk are strong and crowded behind the aperture. 
In the thick edge of the disk, right and left, there are three small, hard cartilages 
of an irregularly oval shape (Plate 18, fig. 1, l.l 2 " i .). 
6. The sense-capsules. 
The eye-balls, which are very small and obscure in the Ammocoete (Plate 8, figs. 
4, 5) are large in the adult (figs. 11-13) ; in the middle species (P. Jluviatilis ) the 
relative size of this organ will be shown in the sections. Unlike the eye-ball of Fishes, 
generally, the sclerotic, even in P. Jluviatilis where the eye-ball is relatively largest, is 
not cartilaginous but fibrous, merely, as in the Mammalia. 
The auditory capsules (Plate 18, figs. 1, 3, 4, au.) are very similar to those of the 
Myxinoids (Plates 9, 10, 16, and 17), being strong, hard, oval capsules, retaining the 
simple form found in the newly-hatched embryo of the Anura. For in these types the 
* In that figure the distal mandibulars are lettered l.l 2 ., 1.1*. 
