OF THE MARSIPOBRANCH FISHES. 
445 
sides the bounding bars of the external branchial opening—on one side directly across, 
and on the other obliquely ; in the former (the left side of the figure) the inturned 
middle part of the main arch is seen for some extent, and then some of its lower part. 
The inner wall of the first pair of branchial pouches is cut through somewhat irregu¬ 
larly, a thing not to be wondered at, considering the size of the fish sectioned. 
1 Oth Section (Plate 25, figs. 5 and 6, 150 and 300 diams.).—This is through the 
third or fourth pouch and the middle of the thyroid involution (th.), at the junction 
of the hind brain (C 3 .) with the myelon. The notochord (nc.) is very large, and 
has no investing cartilage on its sides. Considerable tracts of a pair of extra- 
branchials come into view in this section, which is only deficient by losing the bend at 
each shoulder and the lowest part of each bar in the region of the thyroid body (th.). 
This section shows well what a mere lateral figure (fig. 8) cannot so well, namely, the 
curious inflexion of the main band immediately behind the aperture; this, however, 
is clearly indicated in the figure of the transformed Lamprey’s basket-work (Plate 18, 
fig. 1). This section shows that the junction of the main bar with the upper longi¬ 
tudinal band takes place close to the notochord, but is quite independent of it; 
where the shoulder is cut across (fig. 6), there the rod shows four cells, more or less 
overlapping each other. The other or upper end of the bar (fig. 6) must be considered 
as severed obliquely (see fig. 8), taking in both the longitudinal and the descending 
bar at once ; the edge of the sectioned part passes a little into the interspace between 
the notochord and aorta (nc., ao). 
If anything remains obscure after this description, I hope to make it plain in what 
follows, namely, an account of the cephalic and branchial skeleton of the fully-formed 
and full-sized larva or Ammoccete, 
On the skeletal structure of large larva of Petromyzon fluviatilis, 
In following these descriptions the reader is referred to the external figure also of 
the Sand-pride or Ammocoete (larva of P. fluviatilis, Plate 8, figs. 4-6). 
A.— Vertical section through head and branchial region of a larva 6 inches long. 
In this diagrammatic figure (Plate 25, fig. 10) we see the hooded pouch of the 
mouth (see also fig. 8, m.) formed by the upper lip (u.lp.), with its moss-like growth of 
inner barbels (bb.). Then there is the velum (vl.) separating the oral cavity (stoma- 
d(Eum) from the general pharyngeal space (fore part of archenter on ) ; this velum, or 
right and left vertical curtain, is attached above under the fore part of the auditory 
capsule, and is, so to speak, a production of the inner edge of the hind part ol the 1st 
visceral fold; and behind and external to it, in the re-entering angle between it and 
MDCCCLXXXIII. 3 M 
