ON A FREE-SWIMMING HYDKOID. 5 



The lower part, next to the mouth, bears numerous tentacles 

 of greatly varying size ; these are arranged, not quite 

 regularly, in transverse rows or whorls, and decrease in size 

 from the uppermost whorl, which contains the largest, 

 towards the mouth, around the margin of which the tentacles 

 are very minute. There is altogether a good deal of irregu- 

 larity about the size and shape of these tentacles, and here 

 again one of them was found to be branched (fig. 2, B.T.), 

 but a better idea of their form and arrangement will be 

 gained from the illustration than from any description which 

 I can give. 



Scattered all over the surface of the float, between the 

 bases of the tentacles (figs. 4, etc.), are numerous little 

 branching processes, Avhich we may term "stolons." They 

 branch quite irregularly, their branches remaining short and 

 keeping close to the surface of the hydroid. On these stolons 

 are borne groups of very small medusoids in various stages 

 of development, from minute buds to fully formed bells 

 apparently just ready to separate. 



{h) Internal Anatomy. — The most striking feature of 

 the internal anatomy is the presence of two large cavities, 

 completely separated from one another by a thin horizontal 

 septum, as shown in fig. 5. This septum lies at the level of 

 the junction between the proboscis and the float, and is 

 slightly arched upwards. A preliminary examination reveals 

 the fact that the lower and very much smaller chamber is 

 the main gastral cavity, while the upper one is apparently 

 excavated in the enormously developed mesogloea between 

 the ectoderm and endoderm of the roof of the gastral cavity : 

 this second and much larger chamber I propose to call the 

 "cavity of the float." Its real origin will be discussed 

 presently. It is not a simple cavity, but is subdivided by 

 what I propose to term the "supporting membranes." 

 On the inner surface of the wall of the float there is a net- 

 work of canals, which give it a honeycombed appearance. 

 These canals are lined by endoderm, and are in reality con- 

 tinuations of the gastral cavity, into which they open at their 



