INTRODUCTION. 



the surrounding water. We might be tempted to conjec- 

 ture that the enormous production of spermatozoa which 

 sometimes occurs must be specially connected with the 

 dioscious character of the species, or at any rate that in 

 such cases the fertilization of the ova must be effected 

 from without. But observations on this point are wanting. 

 Joliet states that in the case of several species he has 

 seen the spermatozoa pass from the cell " through the 

 delicate tissue of the tentacular sheath ;" and he supposes 

 that their expulsion may be due to the pressure of the 

 perigastric fluid at the moment of retraction. In other 

 cases we know that there exists a special means of egress 

 the " intertentacular organ," which was first noticed by 

 Dr. Farre*, though he failed to determine its place in the 

 economy. He observed it in Alcyonidium gelatinosum 

 and Membranipora pilosa ; I have also met with it in 

 Alcyonidium mytili and Membranipora membranacea. It 

 consists of an oblong flask-shaped body, which is placed 

 between two of the tentacles, and attached to the tenta- 

 cular ring ; its interior is occupied by a cavity lined with 

 cilia, " which vibrate downwards towards the outer and 

 upwards towards the inner side;" and at the upper extre- 

 mity there is a wide circular orifice surrounded by cilia. 

 There is sometimes a constriction a little below the top, 

 so that the upper portion forms a cup-shaped compart- 

 ment. The organ is closely united to the sides of the 

 tentacles, and is placed on the anal aspect of the body ; 

 below it opens into the perigastric cavity. It is not 

 present on all the polypides in a colony, and is often 



* Philosophical Transact ious for 1837, pp. 408 and 412. See also 

 on British Zoophyte*, 1 ' by the author, Aun.i Mag. Nat. Hist, for NoTcmber 

 1851. 



