4 
well developed hypostome and a circlet of tentacles; the broadest 
part of the hydranth is a little below the latter. The number of 
% 
tentacles is variable; the number most frequently found is 4, but 
there may be as many as 6, or the number may be reduced to 2; 
a few hydranths are even quite devoid of tentacles. The tentacles 
are very short and thick, egg-shaped or nearly globular; the ecto- 
derm is fairly thin and, as far as I can see, destitute of nemato- 
cysts; though the histological structure is somewhat demolished, I 
am convinced that the large endoderm cells leave a hollow space 
in the middle of each tentacle. A thin layer of mesosarc separates 
the endoderm of the tentacles from that of the body of the hy¬ 
dranth; this separation is, however, usually not complete; there 
may be a small opening in the mesosarc, but the lumen of the 
tentacle is never in connection with the gastric cavity. 
The cross-section (PI. I, fig. 4) shows that the unilateral thick- 
ening of the mesosarc, so pronounced in the pedicel, is still indic- 
ated in the hydranth. — In the lower part of the hydranth the 
endoderm is complexly folded, but leaves a fairly spacious stomacal 
cavity (PI. I, fig. 3). The distal part, from a little below the tent¬ 
acles to the mouth opening, is characterized by the mighty. devel- 
opment of the endoderm, which is divided into four longitudinal 
ridges. In this part the endoderm has a much denser character 
than further below and consists of several layers of spindle-shaped, 
radiating cells; near the mouth they are transformed into muscle 
cells. The ectoderm on the lateral sides of the hydranth forms a 
fairly thin epithelium, though with a well developed musculature; 
but around the mouth opening the ectoderm is much thickened and 
provided with a very heavy musculature, but no nematocysts. — In 
short, the hydranth is characterized by the strong development of 
the muscular elements, particularly around the mouth, and by the 
degenerate and rudimentary condition of the tentacles. 
The gonophores (PI. I, fig. 6) are mounted on short pedicels 
springing directly from the stolons. The gonophore is entirely sur- 
rounded by a delicate chitinous perisarc. It has a pear-shaped out- 
line, and its organisation is medusiform. There is a low and broad 
manubrium and four radial canals, but even in the most advanced 
stages observed there are only three marginal tentacles. A fully 
