Appendix to Hydractinia. 
285 
endodermal tubes, and covered above and below by a deli- 
cate investment of chitine. Found on the growing borders 
of the zoophyte, and especially in cuttings of old specimens 
transferred to glass. 
The spines are composed of one tube or many parallel 
tubes: they may be single (PI. XII. fig. 4), and developed 
on a single tube of the polypidom, like those of Podocoryne 
fucicola; single at their summits and of several tubes at 
their base (figs. 5 and 6) ; composed entirely of several (8-12) 
conjoined tubes (fig. 7) ; reticulate by the lateral anasto- 
mosis of their tubes ; or consisting of long ridges of tubes 
reared against each other. 
The polyps spring from one or several confluent tubes of 
the polypary ; they are covered at their origin, and for a little 
distance above it, by a delicate prolongation of the poly- 
pidom. This may be detected by dyeing the whole zoophyte 
with tincture of kino, which gives difi'erent tints to its 
chitinous and fleshy elements, or by steeping it alternately 
in spirit and water, when the coverings of the polyps and 
polypary become inflated as in figs. 2 and 3. 
The polyps are of several shapes and functions, which I 
have described in the paper cited above. It will be sufficient 
to enumerate them here : — 
1. Alimentary polyps, with mouth and tentacles. 
2. Eeproductive polyps, with rudimentary mouth and 
tentacles. 
3. Spiral polyps— a modification of the last; generally 
barren (fig. 3). 
4. Sessile generative sacs of the polypary. 
5. Tentacular polyps, or great tentacles of the polypary 
(flg. 2). 
In the reproductive organs of Hydractinia there is a 
gradual transition from the reproductive polyp to the sessile 
generative sac; the polyp loses its dot-like mouth, its ten- 
tacles, its head or upper part, and finally dwindles down to 
a mere sperm-sac. This change is generally seen in those 
specimens which have long been kept in captivity. In these 
specimens, too, many of the alimentary polyps are often 
converted into large inflated sacs destitute of mouth and 
