﻿248 CONTRIBUTIONS TO THE NATURAL HISTORY 



they are freed from their hydroid polypidom, it has the form of an acute cone, reaching 

 to the very surface of the dome-shaped disk, while, in more and more advanced individuals, 

 it grows broader ; the pointed summit is reduced, and finally this knob appears as a more 

 or less hemispherical mass above the central cavity. This organ is a sort of umbilicus ; it 

 is the remnant of the canal by means of which the Medusa bulb was attached to its hydroid 

 polypedon, and which undergoes the change just described, as soon as the connection has 

 been broken between the maternal stem and the young animal. 



This knob seems to me analogous to that black organ, which, in Beroid Medusae, 

 has been considered as a central eye, or as an organ of hearing, and below which 

 a ganglion is seen. It remains to be ascertained, by embryological investigations, 

 whether, in Beroe, this eye-like bulb is developed in the same manner as the knob of 

 hydroid Medusa?, and whether it undergoes there, after detachment, a higher develop- 

 ment to assume the appearance and functions of an eye-speck. That this may be the 

 case seems probable, when we consider the relation of the two sorts of apparatus in 

 the two types. The upper nervous ring in Sarsia bears the same relation to the central 

 alimentary cavity and to the pigmented disk, that the ganglion and eye-speck of Beroe 

 bear to the chymifcrous system, which opens above its gelatinous disk notwithstand- 

 ing these openings. 



As for the organs of reproduction, I have already mentioned that eggs are developed 

 along the greater part of the proboscis, between the muscular cells, and the external 

 epithelium. These eggs have the same structure as all primitive eggs, being nucleated 

 cells of a peculiar kind, destined to acquire greater independence, and to be cast, after 

 their germinative dot and vesicle have grown to a certain size, and the transparent yolk 

 inclosed in the vitelline membrane has been transformed into a granular, cellular mass ; 

 when they are extruded by the repeated contraction of the proboscis, and are dropped 

 to undergo their independent development. But upon this point I shall enter into more 

 details in another part of this paper. 



One of the most instructive anomalies which I have observed in the genus Sarsia is 

 a modification in the number of parts which I have once noticed in the common species 

 of these shores. Though I have examined many hundred specimens of the Sarsia 

 mirabilis, I have always found it to present the most uniform arrangement of its parts, 

 the specimens having, in every instance, shown four eye-specks, four radiating tubes, 

 and four bundles of radiating muscles, on the outer and on the inner surface of the disk. 

 But, in one instance, two specimens were noticed, among many others, in which the parts 

 were arranged in six (Plate V. Fig. 5); there were six tentacles, six eye-specks, six 

 radiating chymiferous tubes, and six bundles of muscles. The specimens were somewhat 



