PENNATULIDA DREDGED BY H.M.S. "TRITON." 145 



The specimen being in excellent histological condition, I have been enabled 

 to make some observations on the development of the ova. Fig. 35 represents 

 a transverse section through one of the ovigerous septa close to its free inner 

 edge. The septum is seen to consist of a central mesodermal lamella x, clothed 

 on each side by a thick layer of endodermal cells y. Of these cells the super- 

 ficial ones form a layer of short columnar or cubical cells, while the remainder 

 of the endoderm consists of larger cells of irregular polygonal shape, closely 

 packed together, with large granular but rather ill-defined nuclei and granular 

 protoplasm. 



Among these cells certain ones are conspicuous by their larger size and 

 granular appearance, o'. These, which are the germinal cells or primitive ova, 

 appear to arise in the deeper parts of the endoderm layer close to the meso- 

 derm lamella, and as they increase in size gradually move outwards towards 

 the surface. 



Together with this increase of size the ova become spherical in shape, the 

 protoplasm becomes very granular and opaque, and the nucleus, which at first 

 was an ill-defined granular body, becomes vesicular, and acquires a distinct 

 nucleolus and a very well-marked nuclear reticulum. In some of the larger 

 ova o, a reticular appearance is also evident in the protoplasm. 



The ovum, by its continued growth, reaches the surface of the septum, and 

 pushing before it the surface layer of columnar epithelium, which forms the 

 follicular investment, projects freely from the surface to which it remains 

 attached by a short stalk (fig. 34). 



In Sagartia, according to the Hertwigs,* the ova arise in the deeper layer 

 of the endoderm, but sink into and become invested by mesoderm before com- 

 mencing their outward passage towards the surface of the ovary. I have seen 

 no trace of such a mesodermal investment in Umbellula, neither have I seen the 

 peculiar polar fibrillar apparatus described in Sagartia. 



Structure of Zooids. — The zooids of U. gracilis are unique among Penna- 

 tulida, so far as at present known, in possessing pinnated tentacles. 



As shown in fig. 30, the zooids cover all parts of the rachis not occupied by 

 the polyps. The largest zooids are those at the upper end of the rachis, and 

 it is in this situation alone that the zooids with pinnated tentacles occur. Below 

 the polyps the zooids get gradually smaller and smaller. 



Fig. 32 represents on a larger scale a group of zooids from the ventral 

 surface of the upper extremity of the rachis, drawn with the camera. The 

 zooids are seen to be conical, in the best marked cases tubular, projections with 

 a mouth at the free end overhung by a single tentacle, which bears a variable 

 number of pinnules. The pinnules may occur on one side only or on both, 

 and in some cases form a row of five or six on each side of the tentacle. 



* 0. und R. Hertwig, Die Aetinien, pp. 95 seq. 



