PENNATULIDA DREDGED BY H.M.S. "TRITON." 125 



pendent of the leaves, with which they become connate only after they have 

 attained a considerable size. 



The structure of the large zooids, which does not appear to have been 

 examined with any care hitherto, is shown in the series of figures on Plate XXII. 

 Of these fig. 8 represents a median longitudinal section through the whole 

 length of one of the zooids, and through the part of the rachis from which it 

 springs ; while figs. 10 to 16 are transverse sections through a zooid at different 

 parts of its length, fig. 10 being near to the apex and fig. 16 passing through 

 the base of attachment of the zooid. 



The general anatomy of the large zooid is well shown in fig. 8, from which 

 it will be seen that while agreeing in essential structure with the smaller and 

 more typical zooids, it yet presents some points of special interest. 



The zooid is conical in shape, arising by a broad base from the rachis and 

 tapering upwards rather sharply, ending in a pointed apex. As shown in figs. 

 4 and 8 the zooid does not project horizontally outwards, but obliquely upwards, 

 so that we can distinguish between an inner or axial surface, directed 

 toward the rachis, and an outer or abaxial surface facing outwards. 



On the inner or axial surface, and nearer the base than the apex of the zooid, 

 is the mouth (fig. 8, n). This leads into the stomodaeum s, which is lined by 

 columnar ciliated ectoderm cells, the cilia clothing the outer or abaxial wall 

 being of very great length, and forming with the surface of the stomodaeum 

 from which they arise the structure which Mr Hickson has recently proposed 

 to speak of as the siphonoglyphe.* 



The stomodaeum opens below into the general body cavity h, which is lined 

 by endoderm, and is in communication with the cavities of the adjacent zooids 

 and with, the main lateral canals of the rachis, and so indirectly with the 

 polyps. The stomodaeum is attached to the body wall by the usual eight septa, 

 which are well shown in the transverse section (fig. 15). Below the stomodaeum 

 the two septa of the inner or axial surface, bounding the axial interseptal cavity, 

 remain of considerable width, and bear at their free edges the two mesenterial 

 filaments (figs. 8 and 16, p), which are very long and much convoluted, and 

 extend down to the bottom of the zooid cavity. 



The other six septa become reduced immediately below the stomodaeum to 

 very narrow ridges (fig. 16, m), which disappear altogether a short way lower down. 



The body wall of the zooid consists of an outer layer of short columnar ecto- 

 derm cells, below which is the firm gelatinous mesoderm. This latter is much 

 thicker on the outer or abaxial surface than it is on the inner or axial, and is 

 strengthened by a great number of large calcareous spicules (figs. 8-16, i). 

 These spicules are straight rods, thickest in the middle, and with rounded ends ; 



* Hickson, on the "Ciliated Groove (Siphonoglyphe) in the Stomodeeum of the Alcyonarians," Proc. 

 Roy. Soc, 1883. 



