A COLLECTION OF HYDL'OIDS. 345 



'J'lie lateral branches resemble the main stem in structure. 

 Sometimes a joint appears in the middle of an internode 

 i^g.B.j.). 



Diameter of main stem about 1"3 mm, ; diameter of lateral 

 branch, 0*46 mm. ; thickness of perisarc, 0*058 ; length of 

 internode of branch about 1*8 mm. 



Hydro thee a. — Set on a pedicel of 1-3 joints which is 

 inserted on a blunt process at the distal end of the internode. 



Hydrotheca tubular, rather short, expanding, with four 

 equidistant and low teeth, and a four-flapped operculum, two 

 of the flaps are adcauline and two abcauline. Diaphragm 

 massive, situated not far from the base, and excentrically 

 perforated, the aperture being nearer the abcauline side. 



Length of hydrotheca about 1'18 mm. ; breadth at mouth, 

 0'79 mm. ; diameter at the level of upper edge of diaphragm, 

 {)*28 nnn. ; diameter of aperture of diaphragm, 0*20 nnn. ; 

 length of pedicel, 0*13 mm. ; diameter 0*21 mm. 



Hydranth. — About 32 tentacles; nematocysts on tentacles 

 numerous; they measure about 7"2/i in length and 2"7/j in 

 breadth. 



Gonosome. — Unknown ; large ova have been seen in the 

 endoderm at the base of the pedicel in the blunt process of 

 the internode (fig. 38, 0). 



Histology, — The hydrotheca is lined by a sheet of ecto- 

 derm (PL XLYIII, fig. 38, e,.s-.), and on the abcauline and 

 adcauline sides the sheet is provided with two thickenings, 

 beai-ing a series of very large nematocysts (£.n.) arranged 

 horizontally with great regularity. Enlarged views of these 

 nematocysts are given in figs, 39 and 40 ; length 30 ju, and 

 breadth 9 /x. The cnidoblast (en.) and nucleus [nu.) are 

 shown. The nematocysts seem to be ahvays placed so that 

 the ends from which the thread (fig. 38, Th.) is discharged 

 are pointed towards the perisarc cup. It is consequently not 

 easy to see how they can be used; perhaps the cnidoblasts 

 pass up to the edge (fig. 38), in a manner recalling- the 

 succession of teeth of sharks, and are only used at the free 

 edge just below the operculum. 



