PENNATULTDA DREDGED BY H.M.S. " TRITON." 139 



very small and as yet rudimentary polyps, arranged in a manner exactly corre- 

 sponding to the upper sets. 



It is clear that this arrangement might also be described by saying that the 

 polyps are arranged in three longitudinal series, one dorsal and two lateral, the 

 members of each series decreasing in size from above downwards, and this is 

 indeed the method usually adopted. I am disposed, however, to prefer the 

 former mode of description, because it seems to me from an examination of a 

 number of specimens, that the three polyps of each set arise simultaneously, or 

 very nearly so, the dorsal polyps being often a little ahead of the lateral ones, 

 and the right lateral polyps appearing sometimes a little earlier than the 

 left ones. 



There appears, indeed, to be a fair amount of constancy in the arrangement 

 and order of appearance of the polyps. Of twelve specimens, four had 3 polyps 

 only, which were clearly the three of the upper set ; five had 6 polyps arranged 

 as in fig. 23 ; one had 4 polyps, i.e., the 3 of the upper set and the second dorsal 

 one ; and the other two had 4 and 5 polyps respectively arranged in an irregular 

 manner. In specimens with a larger number of polyps than six, it is very 

 difficult to make out any definite plan of arrangement. 



Structure of Polyps. — This has been very fully described by Kolliker,* and 

 will not be considered here in any detail. The main points are shown in the 

 figures 24 to 28. The mesoderm is everywhere, both in stalk, rachis, and 

 polyps, of considerable thickness, and has an immense number of calcareous 

 spicules imbedded in it (figs. 27, 28). Each tentacle (figs. 24 and 25) has 

 along its outer surface a prominent rib, made up of closely packed spicules, 

 while smaller ones extend along the pinnules, as first noticed by Panceri. 



The spicules of the tentacular rib, which may attain a size of 0"66 by 01 1 

 mm., are of the shape shown in fig. 26, and in transverse section in fig. 25. 



The polyps project from the rachis nearly at right angles, as shown in fig. 

 23, and the polyp cavities on reaching the rachis do not stop, but bending 

 down at right angles to their former course, are continued for some distance 

 down the rachis, ending blindly below; the lower part of the stomodseum, and the 

 whole of the organs below the stomodaeum being thus contained within the 

 rachis. The greater part of the thickness of the rachis is, in fact, made up 

 of these lower ends of the polyps, which in a transverse section of the rachis 

 will be seen cut at various levels. 



Fig. 28 represents such a section. On the left side it cuts one of the 

 lateral polyps longitudinally and horizontally (cf. fig. 13) ; on the right side it 

 cuts the corresponding polyp of that side lower down, the section passing 

 transversely through the lower part of the stomodseum. The dorsal polyp 

 of the set is cut at a still lower level, the section passing through the two 

 long mesenterial filaments and the ova. 



* Op. tit., pp. 297-304. 



