HYDEOZOA. 



Fig. 41. 



This distension continues for about a minute, when the upper part, 

 contracting in turn, squeezes back the fluid which fills it into the lower 

 compartment through the opening a, which then closes preparatory to 

 a repetition of the operation. 

 The intervals between these 

 actions were, in the specimen 

 observed by Mr. Lister, very 

 evenly eighty seconds. In 

 Tubularia indivisa the sheath 

 or cell, b, which encloses the 

 polyp is perfectly diaphanous, 

 allowing its contents to be 

 readily investigated under the 

 microscope. When thus ex- 

 amined, a continual circula- 

 tion of particles is visible, 

 moving in even, steady cur- 

 rents in the direction of 

 the arrows (fig. 41,1) along 

 slightly spiral lines represent- 

 ed in the drawing. The par- 

 ticles are of various sizes, 

 some very minute, others 

 apparently aggregations of 

 smaller ones ; some are glo- 

 bular, but they have generally 

 no regular form. 



(195.) The mode of propagation in Coryne, Tubularia, and other 

 genera of Hydriform polyps has occupied the attention of many diligent 

 inquirers, the results of whose labours are extremely interesting and 

 important ; it is, however, to the researches of Professor Van Beneden 

 that science is principally indebted for information upon this subject. 

 According to the observations of that indefatigable naturalist*, the re- 

 production of these zoophytes is effected in no fewer than three different 

 ways : 



1. By continuous gemmation. 



2. By the production of free gemmae. 



3. By simple ova. 



(196.) Observation has moreover shown that in every species pro- 

 pagation is effected by more than one of these modes of reproduction, 

 and sometimes by three or four ; and it must be remarked that in none 

 of them is the cooperation of a male apparatus requisite, neither have 

 any male organs or spermatozoa been as yet detected. 



(197.) Development by continuous gemmation is the simplest possible, 

 * Nouveaux M6moires de PAcademie de Bruxelles, 1843-1844. 



Tubularia indn 



