174 BULLETIN OF THE 
ment. M. Leidyi resembles very closely Bolina alata, Ag. What is very much 
needed is a critical examination and comparison of both genera. Mr. Agassiz 
says the latter genus is limited to north of Cape Cod. 
Mnemiopsis is distinguished from the other Ctenophores, except Bolina, by 
the great development of the lappets or lobes on each side of the mouth, and 
their irregular triangular profile (Plate VIII. figs. 1, 2). The rudimentary ten- 
tacles lie in a groove, covered by a “hood.” resembling a structure of the same 
name in Oyanea covering the otocysts. 
The young M. Leidyi recalls af "leurobrachia in possessing long, flexible tenta- 
cles with secondary appendages (Plate VIII. figs. 3, 4). These tentacles become 
more and more reduced in size with the growth of the young Mnemiopsis, 
until in the adult they reach the rudimentary condition figured in Plate VIII. 
fig. 9. The presence of well-developed tentacles in the young Bolina was first 
pointed out by Prof. McCrady. 
Another likeness between the young Mnemiopsis and the tentaculated Cteno- 
phores, like Pleurobrachia is the development on its aboral pole of a special 
“sense area” of peculiar shape (Plate VIII. figs. 5, 5%), The outline of this 
area as that of the same in the adult Pleurobrachia, is dumb-bell shaped, and the 
otoliths are enclosed in an otocyst, midway between the two extremities. On 
either side of the centrally placed cluster of otoliths, yet within the same sac 
or otocyst, is a single otolith not yet united to the cluster. As the Mnemiopsis 
grows older, the dumb-bell like area of the larva is reduced in size by drawing 
in the two extremities, until, in the adult, it has almost wholly disappeared. 
It seems to be an embryonic sense organ, which is confined to larval 
stages of higher Ctenophores, and to the adult of such lower forms as 
Pleurobrachia. The adult of M. Leidyi (Plate VIII. figs. 1, 2) is very trans- 
parent, when contracted (Fig. 2) ovoid, and when expanded roughly triangular 
in profile. In each lateral hemisphere the walls of the body are continued 
into lappets of great size hanging down on either side of the animal, These 
lappets are very movable, and when the jelly-fish is alarmed they close 
together below the mouth. Their inner walls are crossed by a network of 
muscular lines (Plate VIII. fig. 11) composed of small cells placed side by 
side. The external surface of the body is thickly dotted with small papillæ. 
The oral lappets are separated from each other by deep longitudinal furrows 
along the sides of the body. The diameter of the jelly-fish from the floor of 
one furrow to that of the opposite is much less than that from one surface of 
the oral lappets to the other. In the former of these planes lies the longitu- 
dinal axis of the mouth and the rudimentary tentacles. The length of the 
Ctenophore from mouth to sense bulb is about one half the whole length of 
body and oral lappets taken together. 
In the longitudinal furrows and on each side of the medial line of the same 
lies a single auricular appendage (Plate VIII. fig. 2, h), which arises from the 
walls of the body just above the line, passing through the mouth at right 
angles to the axis of the jell y-fish. These structures extend a short distance 
below the level of the mouth. Their general form is seen in Fig. 2. They 
