Chap. II. 



THE DIFFERENT RADIATA. 



51 



difference of tlie individuals forming these communities, as in the form of tlie 

 basis to which tlie individuals are attached. In Physalia that basis is a sac, 

 inverted upon itself, the inner bag of which, opening externally, is filled with air : 

 the intervening cavity, communicating Avith the open bases of the pendent indi- 

 viduals, contains a greater or less quantity of fluid. Now, suppose the air-bag to 

 be turne<l inside out, there would l>e formed a large and simple hose, containing 

 liquid that may be pressed into the individuals attached to it, or to which the 

 individuals may add by pouring their fluid contents into the bag. In Hj'dractinia, 

 the narrow anastomozing tubes, in the basis of attachment of the polymorphous 

 individuals of the community, may be compared to this hose of the Physalia, only 

 that they are branching. But as a number of individuals arise from each of 

 these stems, we may just as well consider their basis as a single tulie ; and then 

 the only difference Ijetween Hydractinia and Physalia Avould be the narrowness of 

 the tube of the former, and the great Avidth of that of the latter. But reduce 

 the diameter of the one or swell the cavity of the other, and all difference dis- 

 appears, especially if we suppose them both floating or Ijoth attached. It may 

 ])e that the crest of the Physalia, with its many chamljers, carries the homology 

 with the anastomozing tubes ol' the Il3'dractinia still farther. 



As to the various kinds of individuals forming these communities, we find first in 

 Physalia the numerous so-called suckers, or Pol_yps {Figs. 27 h b and 28 h b), correspond- 

 ing to the larger trumpetrlike individuals of 

 the Hydractinia community (PI. XVI. Fiff. 

 1 a, 1 d). These suckers, very numerous, 

 and also much diversified among themselves, 

 are genuine Ilydroids. I have seen them 

 feeding greedily upon small fishes, and 

 gorging themselves to such a degree tliat 

 the silvery scales of their prey could l>e 

 distinctly seen through their distended 



Fia- 



Fkj. 2S. 



■Wl^ 



Bunch of single Hydrre and 

 clusters of Medusae of Phy- 

 salia Arethusa, Til. 

 b b The Uydra?, with their tenta- 

 cles c c. — (Id The bunches of 

 Medusa". 



Bunch of Hydr* of 

 Physalia Aeetiiusa, Til. 

 In various states of contrac- 



walls. But these so-called " Polyps " have tion and expansion. 

 nothing of the polyp structure about them, " Tl^.^Z^^t^l 

 neither radiating partitions dividing their lul'cLftrlf"hrai?Bac''^fri 

 internal cavity, nor tentacles opening di- ang.e H^ar^.-cc Tentace. 

 rectly into radiating chambers, nor an inverted sac hanging in that cavity ; on 

 the contrary, the edge of their oral opening is turned outward as in all Ilydroids. 

 They are, in fiict, Hydroids of the simpler kind, but not so simple as some of 

 the individuals of the Hydractinia communities ; for though they have no whorl 

 of tentacles around their mouth, they have at least one very long and very com- 

 phcated tentacle. Of these tentacles there are two kinds, — larger ones connected 



