REPORT ON THE ACTINIARIA. 117 



The number of the individual polyps and their arrangement on the surface of the body- 

 is almost the same in both the colonies investigated. Eight polyps are uniformly distributed 

 along that circumference which divides the upper half of the shell from the lower when 

 the Pagurus is crawling about on the bottom ; the polyps just mentioned are the largest 

 and most powerful of the colony, and are plainly most favourably placed for acquiring 

 nutriment, as they are always at a little distance from the bottom (PI. III. fig. 2). A 

 medium-sized polyp rises nearly in the middle of the convex upper side of the colony, and 

 in one colony a second smaller polyp lay close beside it. On the lower side an obviously 

 rudimentary polyp grows on the posterior margin of the opening of the shell (fig. 9) ; 

 it has the best position on the lower side, which is on the whole disadvantageous to 

 development, as it is raised from the bottom as long as the Pagurufi is crawling about, 

 and only lies upon it when the Pagurus has retreated into the shell. In the colony 

 consisting of eleven individuals there was the indication of a twelfth between the eight 

 marginal polyps. It may be taken as a general rule that the distribution of the polyps 

 on the surface of the colony is not accidental, but that those spots are preferred in which 

 the animal has room for free development, and also a convenient position for acquiring 

 nutriment. 



The whole surface of the colony is covered by a dirty yellow substance permeated 

 by sand granules, which can be easily scraped ofi"; underneath this the fundamental 

 substance of the wall becomes visible, which resembles cartilage in consistency and 

 colour, and is also hardened superficially by sand granules. This hardened layer is so 

 thin that it can be removed by sections parallel to the surface, and yet leave sufficient 

 fundamental substance both in the coenenchyma and the wall for transverse sections. 

 These conditions were extremely favourable to examination, so that I regretted the more 

 that the colonies were not better preserved. 



The large marginal pol}T3s were 2-2-5 cm. high, 1-4-1 7 cm. broad, and slightly 

 flattened from above downwards. The upper part of the wall is inverted, and forms a 

 horizontal roof ; this might be taken at first sight for the oral disk, as it is separated from 

 the bulk of the wall by a circular furrow, and also diff"ers in its structure, being furnished 

 with numerous (about forty) radial ridges, already observed by Verrill, which are broad 

 where they begin at the margin and become narrower as they run inwards. The radially 

 striated part of the wall is distinguished from the oral disk by being encrusted with sand 

 granules like the rest of the wall. In the middle of this horizontal roof is a fissure 

 running parallel to the margin of the colony, through which, in many polyps, the points 

 of the incompletely retracted tentacles peep out. Through this fissure we may reach the 

 inside of the polyp, first passing through the space lying above the oral disk. 



The fundamental substance of the wall is homogeneous, but in transverse and 

 longitudinal sections it shows a striation parallel to the surface of the body, which looks as 

 if it were deposited in layers (PI. XIV. fig. 5). The striated layers are crossed by fine 



