46 
STRUCTURE AND PHYSIOLOGY 
renewed water. Hence the current within the tubes of some 
polypidoms which has been noticed : it is the movement of the 
nutrient fluid which has found its way from the alimentary sac 
to the surface of the body, where it is subjected to that agent 
which alone can fit it for the purposes of life. 
Amongst zoophytes there is no distinction of sexes, but every 
individual appears to be capable of producing reproductive buds 
or gemmules, or even eggs.* For the production of these, there is, 
in the opinion of some good observers, a peculiar organ or ovarium 
in all the ascidian tribes, and it is certain that their eggs are al- 
ways generated within the polype cell. There are appropriate 
productive organs also in the ITelianthoida and Asteroida, in the 
former situated between the ligamentous dissepiments which ra- 
diate from the mouth to the base, between the stomach and the 
skin ; and in some of the latter attached to the membranous 
dissepiments in the abdominal cavity, while in others the gem- 
mules appear to sprout from every part of the abdominal cavity, 
and of the tube continuous with it. On the contrary, there is 
no local generative organ in any Hydroida — all are 66 full of re- 
productive life in the Hydra germs, similar in all respects to 
the substance of the body, sprout indiscriminately from every 
part of the surface ; in the Tubulariadse they pullulate from 
underneath the tentacula where they may frequently be observ- 
ed in clusters, and, in both of these families, the germs are 
naked or uncovered. But in the extensive family which em- 
braces the Sertularia and all its subgenera, the gemmules, at- 
tached in general to a central placenta, (which is but a continu- 
* “ These corpuscles differ from true ova and seeds, which are ripened by fe- 
cundation, inasmuch as the substance of which the new being is formed is not, 
as ova and seeds are, enclosed in a special envelope, which is separated from 
them at the moment of the developement of the germ, and inasmuch as the 
formation of the new individual is owing to the entire substance of the repro- 
ductive corpuscle.” — Tiedemann’s Comp. Phy. 42 — “ In the present state of 
our knowledge, however,” as Dr Allen Thomson well remarks, “the distinction 
between an ovum and a sporule (or gemmule) must be admitted to be somewhat 
arbitrary.” — Cyclop, of Anat. and Phys. ii. 434. 
In reference to the asexual character of Zoophytes it seems proper to mention 
in this note, that Spix and M. Delle Chiaje consider the Actiniae to be bisexous 
or hermaphroditical, (Blainv. Actinol. p. 79) ; and Raspail has hinted that a si- 
milar doubleness may be the property of the Alcyonella. — Mem. sup. cit. p. 112. 
Nothing has yet been advanced to give these opinions a probable aspect. 
