VESICULARINA '. VESICULARIAD^E. 367 



it requires a practised eye to distinguish them. I have been induced 

 to consider this species as distinct from the genus Alcyonidium, to 

 which it bears a near relation, for the following reasons : 1st. The 

 number of the tentacula, a character which I have found to be con- 

 stant, it being twenty in this and but sixteen in Alcyonidium; 

 2nd. This species never rises from the surface of attachment in the 

 form of an independent polypidom it is invariably encrusting, 

 whereas all the species of the genus Alcyonidium do form elevated 

 polypidoms ; and 3rd. There is a difference in the form of the body 

 or organ to which I have referred in the description of the genus 

 Cycloum, it being circular in this, while it is oblong in the genus 

 Alcyonidium. 



" I have frequently noticed a species of zoophyte lining the inte- 

 rior of old shells of Buccinum undatum, and covering the under sur- 

 face of stones, which I consider to be identical with this. If a por- 

 tion of the polypidom of this species, in a living condition, be sud- 

 denly plunged into spirits, an instantaneous protrusion of the polypi 

 takes place, having their feelers arranged, as in life, in the form of a 

 graceful bell. In this state they may be kept, for a time, for the 

 purposes of future examination. The cause of this protrusion is rea- 

 dily explained. The polypes being already contracted within their 

 cells, on the application of the irritating spirit are compelled to 

 start outwards, the only motion of which they are capable when 

 folded up within these cells." A. H. Hassall. 



IV. VESICULARINA. 



POLYZOA CORNEA, J. E. Gray in Syn. Brit. Mus. 135. 



FAMILY VESICULARIAD^E. 



Since we profess to be guided, in the classification of zoophytes, 

 by similarity in the structure of the polypes considered independ- 

 ently of their polypidoms, the Vesiculariadae, notwithstanding their 

 apparent dissimilarity, must be associated in the same order with 

 Alcyonidium and Flustra. They have been hitherto united with 

 the Sertulariadse; and, previous to our knowledge of their polypes, 

 this seemed a very natural union, for the polypidoms of both are 

 slender and plant-like, horny, fistular, and flexible, and furnished 

 with somewhat similar cells on their branches. But the differences 

 between them even in outward aspect are not inconsiderable; and, 

 although it may be difficult to point out these by any description, 



