268 ON THE GENUS LUCERNARIA. 
Just as in a Medusa-bud, the bell of Lucernaria is sup- 
ported by a stem, which, as the natatory sac does not extend 
into it, consists of only one of the two formative membranes, 
whose existence can be demonstrated throughout the Aca- 
lephe. The Lucernaria attaches itself to various marine 
plants (in the two species examined by me this was always 
Zostera) by the cecal end of this stem, and floats freely in 
the water, mostly downwards, more rarely upwards or in any 
other posture. 
In Lucernaria, consequently, the same disposition of the 
organs obtains as that which is characteristic of the Meduse ; 
and in the following description, therefore, the same terms 
may be employed in speaking of the different parts as are 
commonly employed in the anatomy of the Meduse. 
2. Bell_—The bell consists of the gelatinous disc, constitut- 
ing the outer wall of the cup, and of the natatory sac, which 
forms its inner wall. 
The gelatinous disc (G) is covered outside by the outer 
formative membrane (a), and inside by the inner formative 
membrane (i), between which is a thick layer of gelatinous 
substance (z), which, as in the lower Meduse and Siphono- 
phoree, is quite destitute of cellular elements, appearing to be 
constituted solely of fine, close fibrille, passing for the most 
part straight from one membrane to the other, and which 
may be regarded as mere thickenings in the structure of 
the gelatinous substance. A similar fibrillation occurs very 
generally in the gelatinous substance of the Medusz and 
Siphonophore, and is also apparent in the problematical 
gelatinous substance in the body of the Helmichthyde. 
Both membranes, as in all cases, are composed of a tissue 
of closely contiguous cells. 
At the margin of the cup (fig. 3) the two membranes curve 
inwards and are continuous with the natatory sac (s), the 
intermediate gelatinous substance no longer existing between 
them, and the two membranes, consequently, coming into im- 
mediate contact. It is true that the two cellular membranes 
cannot be demonstrated throughout the whole extent of the 
natatory sac, which appears to consist only of a single layer 
of cells, supporting on its inner aspect a ciliated cuticle, but 
at the line of junction of the gelatinous dise with the natatory 
sac, as well as at the point of attachment of the generative 
organs and the oral tube (fig. 4), they may both be distinctly 
traced, and in the latter situation they are most distinctly 
seen to be separated again by the gelatinous substance. 
At the bottom of the cup the natatory sac is divided into four 
