ON A FEEE-SWIMMING HYDHOID. 7 



transparent, membranous sheets, which radiate outwards 

 from a more solid mass of tissue formed by their union nearly 

 in the middle of the chamber, and which have their edges 

 attached to the inner surface of the wall of the float and to 

 the upper surface of the septum. These remarkable struc- 

 tures I have called the ''supporting membranes." The 

 inner surface of the wall of the chamber exhibits a honey- 

 combed appearance, being marked out into i-oundly poly- 

 gonal areas by the projecting endodermal canals. In the 

 centre of each depressed area between the endodermal 

 canals a knob-like projection may frequently be seen ; this 

 is caused by the tissue which fills the cavity of the tentacle 

 projecting inwards into the chamber of the float like a plug 

 (figs. 6, 8, Ten. PL). These structures we may call the 

 "tentacle plugs." 



The Tentacles. — All the tentacles are filled with a highly 

 vacuolated tissue, composed of sheets or strands of delicate 

 membrane. In the case of the tentacles of the float this 

 tissue may, as just stated, project as a plug into the float 

 cavity. In the proboscis the mesogloea in the wall of the 

 gastral cavity is, in the neighbourhood of the tentacle bases, 

 much thickened and highly vacuolated, giving rise to cavities 

 of considerable size, and this vacuolated tissue is continued 

 into the tentacles (figs. 8, 9). The exact nature and origin 

 of the tissue which thus fills the interior of all the tentacles 

 are, however, by no means easy to determine, and the 

 question will be best dealt with under the next heading. 



(c) Histology. — Pelagohydra exhibits, for a hydroid, 

 a remarkable amount of histological differentiation. For 

 purposes of description it will be most convenient to sub- 

 divide this part of our subject according to the different 

 regions of the body, rather than to attempt to follow out each 

 layer completely before passing on to the next. Indeed, as 

 we shall see later, in some parts of the body the delimitation 

 of the layers is by no means always obvious — at any rate, in 

 the case of the eudoderm. 



As already indicated, the histological preservation of the 



