SEA LAMPREY. 
1185 
is of about the same size as these and of the same 
form. The crescent-shaped posterior dental plate of the 
mouth (pt), ou the other hand, is considerably larger. 
In young specimens it is comparatively higher than in 
old, but also has comparatively smaller and usually 
more numerous (in many cases 8") and more pointed 
teeth. In old specimens the latter (usually 7) are more 
conical, of about the same size and form as the large 
lateral teeth. All these teeth probably serve merely as 
an adhesive organ that holds the Lamprey fast to the 
skin of its prey, which is forced to drag it along. 
Within the mouth is the rasping lingual apparatus that 
opens the skin of the victim and saws into its flesh. 
The anterior linguodental plate (lingual plate, (tin) is 
indeed simple and curved semilunarly like the posterior 
dental plate of the mouth; but at the middle it is deeply 
recurved, thus acquiring the form of a 8. Of the two 
loops thus produced the left bears 7 or 6, the right 6 
or 5 sharp points (tooth-cusps), exclusive of the hind- 
most median tooth of the plate. Here too it seems to 
be the rule that young Lampreys have more numerous 
and more pointed teeth than old * 6 . Behind (within) this 
plate lie the posterior linguodental plates (supralingual 
plates, pin) beside one another, each resembling in form 
a reversed ruminant hoof, with the crook directed for- 
wards and parallel to one of the loops of the anterior 
linguodental plate. The left supralingual plate is usually 
furnished with 13 pointed teeth, the right with 12, 
these being largest in front, on the crook, gradually 
smaller behind. 
The eyes are round. Their size varies considerably 
with age; but as they are covered by the more or less 
transparent skin, the measurements taken depend greatly 
on the transparency of the skin and the method of 
measuring. In Sea Lampreys 4 — 8V 3 dm. long Kroyer 
found the diameter of the eyes to be 4 / 5 % of the length 
of the body or about 7 % of the length of the head to 
the first gill-opening. Our measurements of the outer 
diameter of the iris in so large specimens fix the latter 
percentage at about 10 or 9 C . The opening of the 
nasal duct (the nostril), more or less distinctly ele- 
vated into a tube, is about 1 / 3 as large as the eyes, 
and is situated just in front of the interorbital space, 
at a distance from the tip of the snout measuring about 
2 / 3 (63 — 70 %) of the said length of the head. Behind 
the nasal duct lies the light, elliptical, but indis- 
tinctly bounded epiphyseal spot. 
The gill-openings are comparatively small (see our 
figure in PI. LIII), but when distended they attain a 
height not much less than the diameter of the eyes. 
They are each covered from in front, partly by the 
anterior margin itself, which forms a vertical dermal 
flap, partly by two other, somewhat pateriform, rounded 
dermal lobes, one of which rises from the inner part 
of the lower wall of the gill -opening, the other hanging 
down in a similar manner from above. The hind mar- 
gin of each gill-opening is furnished with small fringes 
and papilla?. The total extent occupied by the seven 
gill-openings in a row one after another measures about 
9 or 10 % of the length of the body, and in young 
Sea Lampreys (2 — 3 dm. long) is in many cases equal 
to the length of the snout to the nostril or about 2 / s 
of the length of the head to the first gill-opening d , in 
the old greater, about 4 / 5 (77 — 84 °%) of the said length 
of the head, which in the young is up to 15 %, in the 
old only about 11%, of the length of the body. 
The first dorsal fin is an elongated, low triangle, 
with the greatest depth (7 5 — ’7 of the length) situated 
in the anterior part, but with the anterior end indis- 
tinctly delimited from the dorsal edge of the body, 
which margin in ripe individuals (oftenest in males, 
but sometimes too in females) may rise into a hard 
carina both before the first dorsal fin and between it 
and the second dorsal. This renders it also difficult 
to determine with certainty the distance between the 
first dorsal and the tip of the snout*, but the rule 
appears to be that the fin commences close behind or 
even at the middle of the body, comparatively further 
a According to Meek (1. c.) exceptionally 9. 
6 Blunter and sharper teeth thus occur in this species as in the following one. In estimating the importance of this difference, which 
is by no means exclusively dependent on age, it should be remembered that we have here to deal not with real teeth, but only with points 
of horny sheaths, which are changed time after time. In ordinary cases the horny sheath is so loosely attached to the plate that it can be 
removed altogether, new, unworn, and therefore (if they be fully developed) more pointed teeth being thus exposed to view. 
c Lilljeborg gives 9 or 8 % (*/,, — '/i 2 )- According to Meek (1. c.) this percentage is about 16 — 13 in Sea Lampreys 2 1 / 2 — 3'/ 2 
dm. long from Cayuga Lake. 
d According to Meek (1. c.) sometimes only 55 % of the last-mentioned length. 
e According to Meek this distance varies with great irregularity between 55 V 2 and 49 % of the length of the body. 
