230 THE OCEAN WORLD. 



of Geneva, from whose work on the "Inferior Animals of the 

 Mediterranean" our details are borrowed. F. spirans, sometimes 

 called F. Iwibosa, was discovered in the Mediterranean, between 

 Monaco and Mentone, by Forskahl, who most erroneously took it for 

 a holothuria. On the upper surface of the animal is a hydrostatic 

 apparatus, the object of which is to maintain its equilibrium in the 

 ambient element. This apparatus consists of a shield and a crest, 

 organs of which M. Yogt gives a very detailed description ; but it is 

 on the under surface that the principal organs of the Yilella are 

 exhibited. These are not seen when the animal swims, because 

 under such circumstances the vertical, oblique crest only is visible. 

 The lower surface is concave, with a sort of mesial nucleus, 

 presenting at the extremity of a trumpet-like prolongation, whitish 

 and contractile, a sort of central mouth, surrounded by tentacular 

 cirri, the external row being much longer than the internal ones. 

 This was formerly thought to be the stomach of the Yilella. In the 

 present day, this appendage is known to be the central polyp around 

 which are grouped other whitish and much smaller appendages, the 

 base being surrounded by little yellow bunches. These are supposed 

 to be the reproductive organs. Between the crest and the shield 

 numerous free tentacles present themselves, vermiform in appearance, 

 cylindrical, and of a sky-blue colour, which are kept in continual 

 motion. 



The Yilella is therefore not an isolated individual, but a group or 

 colony, in which the individuals intended to be reproductive are the 

 most numerous, and occupy the inferior parts. 



The central polyp, by its size and structure, is distinguishable at 

 the first glance from all the other appendages of the lower surface of 

 the body. It is a cylindrical tube, very contractile and pear-shaped, 

 swollen into a round ball, or considerably elongated. Its mouth is 

 round and much dilated ; it opens in the cylindrical or trumpet part, 

 which is contained in a sac in the form of elongated fusci, clothed in 

 the whitish integuments which formed the body of the polyp when 

 perfect. At the bottom of the sac two rows of openings are observed, 

 which lead to a vascular network extending over the whole body ; the 

 membranous parts, while affecting various conditions in their arrange- 

 ment, are nevertheless in direct communication with all the reproductive 

 individuals. 



