190 ERNEST WARREN. 



about 18'7 /Li ill thickness, and is obviously lamiiiiited in 

 structure. The stem carrying the hydranth has a perisarc 

 consisting' of two layers : (1) An inner laminated layer con- 

 tinuous with the perisarc of the stolon (PI, XXXIII, fig. 3, ch."), 

 and (2), an outer vertically striated layer {ch}). This layer 

 alone forms the cup of the hydranth, and its upper edge is 

 inserted in the shallow perisarc-groove just below the verticil 

 of tentacles. The outer layer gradually thins out towards the 

 base of the hydranth-bearing stem, while the laminated layer 

 thins out and disappears at the base of the hydranth. The 

 thick perisarc around the gonophore consists of the laminated 

 layer only (PL XXXIII, figs. 3 and 5, ch. 2 ; text-fig. 3, 

 B, cJi.~). The whole of the perisarc is apparently covered 

 with a gelatinous or sticky substance, as there is normally a 

 very conspicuous coating of small particles of mud and sand. 



In the developing hydranth the whole of the ectoderm of 

 the cup-like base appears to take part in the secretion of 

 perisarc. The perisarc-groove, where the perisarc terminates, 

 is a more obvious structure than in the mature hydranth 

 (text-fig. 2, B, p. g). It would appear that the outer 

 vertically striated layer {ch.^) is only secreted by the 

 hydranth, and that as the hydranth elongates and forms a 

 stalk, this layer of perisarc is, so to speak, left behind, and is 

 then strengthened by the secretion of the inner laminated 

 layer by the general ectoderm of the stem. It may be noticed 

 that in the majority of the gymnoblastic hydroids the groove 

 or " collar " occurs at the top of the stem bearing the polyp, 

 where the perisarc terminates ; while in the present species it 

 occurs just below the verticil of tentacles of the hydranth. 



Ectoderm. — The histology of the ectoderm is typical. 

 The nematocysts are of one kind only in the hydranth, and 

 are somewhat small, measuring about 5*0 ^ in length and 2-9 fj. 

 in breadth. I have not found them occurring in the endo- 

 derm. They are scattered somewhat sparsely throughout the 

 ectoderm of the tentacles and general coenosarc. In the 

 gonophores, nematocysts are abundant, and they tend to be 

 somewhat larger than those on the tentacles (5*6 nt in length) ; 



