ON A FBEE-SWTMMING HYDROID. 15 



proboscis tentacles, embryological research may be required 

 before we can say exactly how they arise. 



The Stolons. — The stolons are simply branching hollow 

 outgrowths of the wall of the float in the neighbourhood of the 

 endodermal canals, which are prolonged into them to their 

 extremities (figs. 8, 12, St.). The ectoderm (fig. 12, Ect.) is 

 composed of the usual large clear cells, rectangular in 

 longitudinal section, with small nuclei pressed against their 

 dividing walls. At its base lies a feebly developed layer of 

 longitudinal muscle-fibres. Thread-cells are almost entirely 

 wanting. The mesogloea is thick, and traversed by slender 

 threads crossing from ectoderm to endoderm. The endo- 

 derm (fig. 12, End.) is simply a continuation of the endoderm 

 which lines the outer walls of the endodermal canals, and, 

 like the latter, is composed of large cells, often with rounded 

 extremities projecting into the central lumen, with enormous 

 vacuoles and darkly staining contents massed together either 

 in the rounded end or elsewhere. They have small nuclei, 

 and in addition contain darkly staining spherical globules of 

 various sizes. 



The Thread-cells.— The thread-cells (figs. 16, 17) are of 

 large size. The actual neraatocysts or capsules are approxi- 

 mately ovoid in shape, but truncated at the somewhat 

 narrower outer ends, and measure, when fully developed, 

 about 0*0128 mm. in longer diameter. Each one is more or less 

 enclosed in a delicate cnidoblast (fig. 17, cnh.). When fully 

 developed the thread-cells lie in the outer parts of the large 

 ectoderm cells just beneath the surface, and the cnidoblast is 

 prolonged inwards to the base of the cell in the form of a long 

 thread — the cnidopod' (figs. 16, 17, Cnjp.). The cnidopod is 

 remarkably distinct and tough, so much so that when the 

 ectoderm of a tentacle has been abraded, so that the large 

 ectoderm cells have disappeared, the cnidopods may still 

 remain projecting from the surface like hairs, with or with- 

 out the thread-cells still attached to their extremities. 



1 Compare AUnian, 'Cliallenger Reports,' " Hydroida," Part 2, p. xv, for 

 tl)e use of this term. 



