42 THE VOYAGE OF H.M.S. CHALLENGER 



condition into a plate-like expansion, from the indented centre of whicli run 15-20 

 furrows towards the thickened edge. 



Habitat. — Station 49, May 20, 1873; 83 fathoms, upon a Gastropod shell 

 tenanted by a Pagurus, the shell entirely dissolved away by the cosnenchyme. 



Dimensions. — Length of the polyp, 0-6-1 '0 cin. ; breadth, 0-3-0-5 cm.; colour, 

 greyish-yellow. 



"This species forms a colony of eleven individuals, on a shell some 2-0 cm. high. 

 The calcareous substance of the latter is completely absorbed, and at all points replaced 

 by the coenenchyme, the latter ha^dng obviously taken its place, while preserving its 

 'external form. Only the anterior side of this coenenchymatous structure, i.e. the part 

 directed forwards in movement of the Crustacean, possesses polyps ; the free posterior 

 side allows the coils of the former Gastropod shell to be clearly recognised. Of the 

 eleven individuals, eight large mature polyps occupy the edge of that side which is 

 directed anteriorly in the movement of the crab. They form long cylindrical tubes, 

 6-10 mm. high and 3-5 mm. broad. In the median space which they bound, stand 

 three very young polyps, projecting as vei-tical cylindrical warts from the coenenchyme, 

 with height and breadth alike of r5-2 mm. One may remark that the large polyps 

 bend forwards, i.e. their oral discs face upwards, in the direction corresponding to the 

 locomotion of the Pagurus, so that they are most favourably placed for the reception 

 of the food matters which stream against them. Owing to the curving just mentioned, 

 the large polyps are above stroirgly compressed laterally. 



"The whole colony has a rough shagreen-like exterior, of a grey colour. The 

 otherwise smooth body-wall forms above a horizontal plate, which not only projects 

 like the capital of a column over the vertical part, but has also a characteristic sculp- 

 ture, and the appearance of a plate with raised edges and indented centre ; in the 

 middle of the latter lies the entrance to tlie interior, which is slit-like, corresponds 

 to the lateral compression, and is always recognisable as an obvious opening. From 

 this median point outwards radiate over the plate-like surface 15-20 radial furrows, 

 which are continued outwards for a short distance over the marginal thickening, 

 appearing on it as deep notches. 



" When a polyp is opened with scissors, one remarks that the mesenteries run 

 -down the whole length of the body-wall, but do not pass over on to the horizontal 

 floor of the ccelenteron. In the lowest parts of the polyp, the mesenteries are visible 

 as slightly projecting ridges, striking the eye by their clear colouring; at about one- 

 fourth of the total height, the macromeseuteries form filaments ; these are yellowish- 

 white contorted coils, which completely obscure the micromesenteries. One can 

 without damage remove the mesenteries from the body-wall, and study them indepen- 

 dently. The supporting lamina of the mesenteries is very thin, and runs simply to- 

 the base without any excavation ; the mesenterial filaments are of the customary 



