A NEW FAMILY OF HYDROIDEA. 125 



perisarc, as seen in a very thick section cut transversely to the length of a branch. 

 This operculum-like structure is very thin and liable to be torn away, but is present 

 in all well-preserved and cut specimens. Sometimes it projects as a stiff collar, 

 sometimes (Figs. 4, 16) it is thrown into folds, and at others is withdrawn into the 

 hydrotheca. With only specimens in which the polypes are completely retracted to 

 examine, I cannot say how much of the body of the polype is covered by this thin 

 collar when the animal is fully expanded. 



When the soft parts are dissolved away by potash the whole branch appears to 

 consist of a meshwork of tubes with chitinous walls (Fig. 11), which anastomose so 

 freely that the skeleton appears, in surface view, to consist of an irregular chitinous 

 network, which has a considerable superficial resemblance to the skeleton of a horny 

 sponge, though this resemblance is even more strongly marked in the case of the 

 Ceratelladse. On cutting sections, however (Figs. 7, 8, 9), the tubular arrangement can 

 be recognised. The structure is essentially identical in the branches and branchlets, 

 whatever be the size, the only difference consisting in the greater number of tubes 

 entering into the composition of the larger branches as compared with the smaller 

 ones. The tubes (Fig. 7) run roughly parallel to the length of the branch, continually 

 branching and anastomosing. The spaces which they contain vary much in calibre. 

 Towards the exterior the chitinous walls are much stronger and thicker than in the 

 interior, and the most external series are incomplete, forming grooves rather than 

 tubes. The thin external layer of the branch, previously alluded to, touches the 

 chitinous lips of these grooves (Figs. 8, 9), and thus a complete inclosure for the most 

 external-lying ccenosarcal tubes is formed. 



Sections also show that the larger circular openings, visible on the outside, lead 

 down into tubular spaces, the walls of which are formed of chitinous material similar 

 to that of the tube walls with which they are connected. These form the hydrotheca, 

 and the spaces within them are continuous with one or more of those within the 

 tubes surrounding them (Figs. 4, 7, 12). The thin external chitinous (?) layer always 

 passes up to be attached to the rim of the hydrothecae. The cylindrical structures 

 (P") on this thin layer are very simple in form, open at both ends, and serve during 

 life to contain the defensive polypes which are directly connected with the external 

 layer of ccenosarcal tubes. (Fig. 5). 



Measurements of the Skeleton. 



Diameter of branch taken, "7 mm. 



Number of tubes, as seen in longitudinal section, entering into the 

 composition of a branch of this size, 7 — 9 ; as seen in transverse 

 section, 40 — 50. 



Thickness of the tube, varying, but averaging about "07 mm. 



