192 THE VOYAGE OF H.M.S. CHALLENGER. 



They are simple iu one family only, the Apolemidse. In all the other families they bear 

 a series of equidistant lateral branches or tentilla. These are rarely quite simple secondary 

 filaments (Circalia). Usually each tentillum is divided into three portions, a basal 

 pedicle, an intermediate cnidoband, and a terminal filament ; this latter is either simple 

 or multiple. The greatest variety of structure and form is exhibited by the middle part, 

 the cnidoband (sacculus, " Nesselknopf "), and this presents the chief characters for the 

 distinction of genera. The basal pedicle is always a simple cylindrical tubule, often 

 dilated or vesicular at the distal end. 



The cnidoband or cnidonode (cnidosac, sacculus, "Nesselknopf"), the middle and most 

 important portion of the tentillum, is originally nothing more than a dilatation of the 

 middle part of the simple filiform and tubular tentillum, produced by the unilateral 

 development of larger cnidocysts in its wall. Then follows the dislocation of the 

 central canal, which becomes excentrically placed ; next a bilateral form, and soon a 

 spiral twisting of the dilated portion. That side, in which the excentric canal runs, is 

 the ventral side of the cnidoband, the opposite, in which the cnidobattery (or the accumu- 

 lation of larger cnidocysts) is placed, is the dorsal side. Between them is developed the 

 elastic " angleband," a group of two or four parallel elastic ribands. The excentric canal, - 

 also more or less coiled, runs in the axis around which the cnidoband is twisted. The 

 spiral is always a left-handed, or lambdoidal. The cnidocysts which compose the cnido- 

 battery are usually of two kinds, very numerous small and paliform, and a smaller number 

 of large ovate or ensiform thread-cells ; the latter are arranged usually in two lateral rows 

 on the proximal part of the cnidobattery. 



The spiral cnidoband is usually coiled up in three or four left-handed turnings ; but 

 sometimes it makes six to eight turnings or even more. It remains naked, without 

 involucrum, in Athoria (PL XXI. fig. 8), Halistemma, the Forskalidre (PI. X. fig. 23) and 

 in part of the Anthophysidse (Rhodophysa). In the majority of the Physonectse there 

 is developed around its proximal end a peculiar involucrum, covering it like a cap or 

 hood ; it is a solid annular fold of the exoderm, which arises from the distal end of the 

 pedicle and grows distally. Claus maintains that this envelope " evidently corresponds 

 in morphological relation to the umbrella of a Medusa " (74, p. 2) ; but their likeness 

 is merely external. The involucrum is nothing more than a simple protecting mantle for 

 the naked cnidoband. At first it envelops the proximal base only (Stephanomia, 

 Cupulita, Anthemodes, PI. XV. fig. 11, &c), but afterwards it grows around the entire 

 cnidoband and envelops it distally as an external capsule. The most complicated structure 

 is exhibited by the involucrum in the Discolabidse ; where it often bears a pair of red 

 lateral ocelli (PL XX. fig. 14) ; the convex dorsal side is here so strongly developed, 

 prolonged and much twisted, that the cnidoband is recurved and its distal end placed 

 near the proximal base on the contracted ventral side ; the terminal filament becomes 

 reduced in this case and finally disappears. 



