12 THE OCEANIC HYDROZOA. 



dilatation — the basal sac. The tentacle itself is a filament which in a large specimen attains 

 the length of many feet when fulh^ extended. It has no lateral branches, but it is beset along 

 one side, at regular intervals, with reniform enlargements, full of large thread-cells, which are 

 disposed transversely to the axis of the tentacle, and look like so many beads threaded upon 

 it. On the opposite side, the tentacle widens out into a ribbon-like muscular band, which, 

 attached above to one edge of the basal sac, is the agent of its rapid and extensive contrac- 

 tions. A canal traverses the whole length of the tentacle, and sends caecal diverticula into the 

 reniform enlargements, while above, it communicates with the cavity of the basal sac. It has 

 been supposed that the latter organ, by its contraction, drives the liquid which it contains into 

 the canal of the tentacle, and thus effects its elongation. Without denying that such may 

 be its office, I would remark, that the tentacles of other species which are not provided with 

 basal sacs are just as capable of rapid elongation. 



The reniform enlargements to which I have referred may be regarded as rudimentary 

 lateral branches. If they be supposed to elongate and become filamentary, the result will be 

 a tentacle very similar to that possessed by RMcojjJii/sa, except that in this genus there is 

 no basal sac, nor muscular band, and that each tentacle is attached to a polypite. The lateral 

 branches of the tentacles of MUzophysa have one wall much thicker than the other, but 

 it contains only spheroidal thread-cells, and the branches are not divided into distinctly 

 characterised regions. 



In Forskalia, however, while the tentacles have essentially the same structure, each lateral 

 branch is divided into three distinct portions: a proximal slender part ; a median division, 

 with one wall much thicker than the other, containing numerous elongated thread-cells, 

 arranged in transverse rows perpendicularly to the wall, and flanked on each side by a longi- 

 tudinal series of larger oval thread-cells ; and, finally, a terminal cylindrical thread, full 

 of small, rounded thread-cells. I shall term the first of these regions the jiedicle, the second the 

 sacculus, and the third \he. filament. The muscular band of Plnjsalia is partially represented 

 by two pairs of muscular cords, which, according to Leuckart(Z. N. K., p. 99), lie in the thin 

 wall of the sacculus. In the contracted state, the sacculus and the filaments are thrown into 

 spiral coils. 



The structure of the tentacles of Halistemma is essentially the same, except that they 

 are provided with a more complex muscular apparatus, for a description of which I must refer 

 to the works of Vogt and Leuckart. 



The tentacula of the CaJycophonda (PI. V) resemble those just described, and arise cither 

 from the base of the gastric division of the polypite, close to the pedicle, or from the latter itself. 

 The larger, oval thread-cells are confined to the distal end of the sacculus, which is usually 

 bent so as to have a half-moon shape, the thick wall forming the convexity. The filament is 

 coiled up into a close spire, folded against the straight, thin wall ; and, where it joins the 

 sacculus, the points of five or six oval thread-cells commonly project, like those of the rowel of 

 a spur. Where the peduncle joins the sacculus it exhibits a small dilatation, which I 

 conceive to be a rudiment of a part to be described presently as the involucrum. 



Leuckart has particularly described a structure in the sacculus of the Cali/cophorida, 

 where it has also been noticed by Vogt and Kolliker, which he terms the "angelband." 

 It is " a simple, but strong and sharply defined, muscular cord, which is folded in zigzags, 

 and lies in the posterior (thin) wall of the canal of the sacculus, partly covers it at the 



