PHYSALIA. 103 



remarkable for their deep blue colour, which is not merely confined to the reniform elevations, 

 containing thread-cells, with which they are beset, but extends in a less degree on to 

 the stem of the organ. 



Each tentacle consists of two very distinct parts — the short, broad, basal sac (c"), and 

 the filiform tentacle proper (c^) (fig. II). 



The former is a sort of conical bag, attached by a broad base, and terminating in a free, 

 pointed apex, which is nothing more than a csecal process of the hydrosoma, containing a 

 diverticulum of the somatic cavity. Its inner surface is ciliated. The outer presents manv 

 thread-cells, whose number increases towards its base. 



On one side, the edge of the sac is rounded and thick ; on the other, it th.ins and 

 expands into a broad, muscular membrane, which is continued dovs'n into the muscular band 

 of the tentacle. 



The tentacle proper has a common base with the sac, but is otherwise separate from it. 

 It is essentially a caecal tube, whose canal coinnumicates with the cavity of the sac and with 

 that of the hydrosoma, by means of the common base. On the side turned away from the 

 basal sac, the ectoderm of the tube is much thicker than on the opposite side, where it 

 expands into a thin muscular band, continuous above with the muscular expansion of 

 the basal sac. The endoderm of the tentacle is ciliated. 



The proximal end of the tentacle is gathered up into folds, which are attached to 

 the edge of the tentacular sac by the muscular membranous expansion, just as the 

 folds of an intestine are held by a mesentery. The rest of the tentacle hangs down 

 straight. 



At the attached end of the convoluted part of the tentacle, the thickened wall was quite 

 even and smooth, but a little lower down, the wall had become sacculated (fig. 1 1 a), 

 each sacculation involving both the ectoderm and the endoderm, and containing a short, broad 

 caecum of the tentacular cavity. At first, the sacculations involve only a small segment of the 

 tentacular wall, but by degrees they extend transversely until, at length, they leave between 

 their extremities only suSicient space for the longitudinal band of muscular fibres, into which 

 the ectoderm of the opposite wall of the tentacle is metamorphosed, and for the tentacular 

 canal, which, within the cavity of each fold, expands into a caecum, corresponding with the 

 fold in shape. Throughout the convoluted part of the tentacle the folds are very close 

 together ; but, in the beginning of the straight portion, they separate from one another, 

 and throughout the rest of its length thej'' are, in the uncontracted state of the organ, 

 a long way apart. 



When fully formed and extended, consequently, the tentacle looks like a delicate, narrow 

 ribbon, having transverse, reniform thickenings at regular intervals (fig. 12). The substance 

 of each thickening has a dark blue colour, and imbedded within it are myriads of close-set, 

 colourless, spherical thread-cells, varying from 34th of an inch in diameter, to a sixth of 

 that size. Under a low power a number of fine filaments, the extruded threads of many of 

 these cells, might be seen radiating from the surface of each reniform thickening. 



The small tentacles arise indiscriminately among the appendages of the rest of the 

 hydrosoma ; they precisely resemble the larger ones in structure, but they are, in addition 

 to their small size, distinguished by their pale colour, the blue coloration being either absent 

 or entirely confined to the reniform bodies. 



