17 



expansion. The fibres are very distinct in nearly every part, but are 

 especially strongly developed towards the summit of the bell and to- 

 wards its lower margin : they are arranged in a circular direction, and 

 I have not been able to observe the four distinct longitudinal bands of 

 muscle described or rather figured by Prof. E. Forbes. The muscu- 

 lar fibres, or fasciculi, vary from 1-5%^ to T 6 x do of an inch in width, 

 and, as in the higher animals, are composed of smaller fibres or 

 fibrillae, distinctly marked with regular transverse striae (fig. 8, a). 

 The spaces between the muscular fibres are comparatively large. 

 The border of the bell is somewhat thickened, containing the 

 marginal canal, and to its lower margin are appended the tentacles 

 and bulbs, or rudimentary tentacles. Attached to the inner margin 

 of the border is the narrow and inconspicuous velum, which is to be 

 regarded perhaps as a prolongation or continuation of the subum- 

 brella, and like that part is composed almost entirely of concentric 

 muscular fibres ; it is the proper sphincter or contractor of the ori- 

 fice of the bell. The deficiency in size of the velum is in some degree 

 compensated by the greater development of the muscular structure 

 of the subumbrella itself. 



The inferior margin, as above stated, is beset with a close and 

 crowded series of rounded or pyriform bulbs, all of which present a 

 circular, crimson spot towards or at the lower part, and a more faintly 

 coloured, somewhat elongated, yellowish brown spot towards the 

 upper part. The tentacles arise from these bulbs, and when com- 

 pletely retracted appear to constitute the narrow pyriform prolonga- 

 tion exhibited by some, but not by all of these bodies. The tentacles 

 vary very much in number : in a full grown specimen, and when all 

 are fully extended, they are very numerous ; in fact so much so as 

 not to admit of being accurately counted. In length they exceed, 

 perhaps, twice or three times the height of the bell, and form, when 

 in the extended condition, a beautiful light fringe or halo around the 

 animal as it sinks slowly in the water. When the creature ascends 

 or swims about by the contractions of the disk, the tentacles are al- 

 most invariably drawn in entirely, or at all events much shortened. 

 The bulbs are about ^\^ or -j^tr of an inch in diameter, and the ten- 

 tacles considerably less. The bulbs and tentacles are constituted of 

 a thin transparent membrane, the interior of which is filled with a 

 delicate cellular tissue (fig. 12). The outer layer of cells of this 

 tissue, have in the bulb something of a prismatic form. In the ten- 

 tacles the shape of the cells is considerably altered, they become 

 narrow and elongated, their long axis (in the contracted state of the 



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