174 BULLETIN OF THE 



nient. M. Lcidiji 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 C'yanea covering the otocysts. 



The young AI. Leidrji recalls a Pleurobrachia 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 jonng 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 Avails 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 papillae. 

 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 jelly-fish. These structures extend a short distance 

 below the level of the mouth. Their general form is seen in Fig. 2. They 



