NARRATIVE UF THE CRUISE. 
575 
the Paractiniae and Monaulese the septa are grouped in pairs, some number other than 
6 is the basis of their arrangement. 
“ One Paractinia, Sicyonis crassa, certainly, and another, Polyopis striata, probably 
belong to the tetramerous Actiniae, and probably bear to the fossil Tetracorallia a relation 
similar to that which the hexamerous Actiniae bear to the recent Hexacorallia. 
“ Scytophorus striatus, the sole representative of the family Monauleae, possesses an 
uneven number of pairs of septa, because in it there is wanting one of the two pairs which 
correspond to the extremities of the sagittal axis, and are called ‘ directive septa ’ from 
their peculiar position and structure. 
“ It is of interest to observe that in the Actiniae, as in other groups, types of structure 
Fig. 189 . — Pobjsiphonia tuberosa, Hertwig ; A, oral view, and B, lateral view. Station 235, 4th June 1875, off Japan, 
565 fathoms ; natural size. 
which have died out in shallow water have maintained themselves in the deep sea. 
Still more striking is a second peculiarity of the Challenger collection, which is also 
explained by the fact that all the specimens come from great depths ; it contains 
numerous forms in which the tentacles have undergone a more or less extensive retro- 
grade metamorphosis, by means of which they become converted first into short tubes 
and finally into simple openings surrounding the mouth. This modification can be 
followed step by step in different species. Paradis tubulifera (1875 fathoms) has still 
well-developed tentacles, like most Actiniae, except that the terminal aperture, which 
in these is quite small or even entirely absent, has expanded into a widely gaping orifice. 
(narr. chall. exp. — vor. i.— 1885.) 73 
