4() STRUCTURE AND PHYSIOLOGY 



renewed water. Hence the current within the tubes of some 

 polypidonis which has been noticed : it is the movement of the 

 nutrient thiid which has found its way from the ahmentary sac 

 to tile surface of the body, where it is subjected to that agent 

 which alone can fit it for the purposes of Hfe. 



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 aU 

 ways generated within the polype cell. There are appropriate 

 productive organs also in the Helianthoida 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 " 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 Tubulariadae 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 r.ot, 

 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." — Ticdemann's Comp. Phy. 42 — " In the present slate of 

 our knowledge, however," as Dr Allen Thomson well remarks, " the distinction 

 between an ovum and a sporule (orgemmule) must be admitted to be somewhat 

 arbitrary." — Cyclop, of Anat. and Phys. ii. 4-34. 



In reference to the asexual character of Zoophytes it seems proper to mention 

 in this note, that S\n\ and M. Delle Chiaje consider the Actiniae to be bisexous 

 or herma])hroditical, (Blainv. Actinol. p. 79) ; and Raspail has hinted that a si- 

 milar doublencss may be the property of the Alcyonella — Mem- sup. cit. p. 112. 

 Nothing has yet been advanced to give these opinions a probable aspect. 



