70 THE VOYAGE OF H.M.S. CHALLENGER. 



(2) liexact liypodermalia ; (3) pen tact hypogastralia ; (4) oxyliexasters scattered among 

 the parenchyma ; (5) protuberant floricomes. 



The spicules which belong to the first category, and are provided with thick, short, 

 conical rays, are scattered in the inner portion of the walls of the tube. They are 

 particularly abundant, and are arranged in a cruciform manner in the circular membrane 

 surrounding the parietal gaps. In this situation peutacts chiefly occur, in which the 

 unpaired ray penetrates the parenchyma radially outwards, while the four rectangularly 

 crossed rays of the two other axes lie parallel to the bounding surface. Often, however, 

 (especially in the inner thinner marginal portion of the circular membrane), only one 

 of the two latter pairs of rays is fully developed, so that triacts arise whose paired rays, 

 lying in the same axis, extend tangentially to the free margin of the parietal gaps, while 

 the unpaired ray is directed radially. If, again, the latter remain undeveloped, diacts 

 arise which have been designated by Marshall " compass-spicules " on account of their 

 resemblance to a compass needle, being much swollen in the middle. Completely formed 

 hexacts belonging to this category of spicules are less frequent ; when they occur they 

 lie in the parenchyma at some distance from the gastral surface, or from the free margins 

 of the parietal gaps. They are usually regularly developed (PI. III. fig. 18), more rarely 

 shortened in one ray (PI. II. fig. 1, a.b.c). While in the lattice-like network of the 

 outer skin dermalia proper are absent, regularly arranged liypodermalia occur 

 underneath as slender hexacts of equal form and size, and provided with a greatly 

 prolonged proximal ray, four times longer than the other five rays, which are of 

 approximately equal length, and like the former run gradually out to a point (PL III. 

 fig. 16 ; PI. IV. figs. 3, 4, 5). The long principal axis is at right angles, the two 

 transverse axes are parallel to the surface of the body. 



In well-preserved portions the axial cross of the hypodermalia lies about O'l mm. 

 beneath the skin. Their distal ray extends into a point-like elevation of the skin, which 

 it seems to push out. On its outer end it carries a floricome which extends over the 

 skin (PI. IV. fig. 4). 



Since the corresponding tangential rays of the neighbouring hypodermalia are apposed 

 to one another laterally for half of their length or even more, quadrate or rectangular 

 meshes are formed, over the centre of which the skin is, as a rule, somewhat depressed 

 (PI. IV. fig. 4). If this involution extends still further, the skin finally becomes applied 

 closely against the tangential rays of the hypodermalia, and it may even seem as if the 

 latter occurred in the skin. On the summit of the outer ridge there is usually a row of 

 particularly strong and long hypodermalia. 



To the system of the hexact hypodermalia there corresponds, on the inner side of the 

 entire tube and of the large excurrent passages, a system of similar slender pentact 

 hypogastralia with pointed extremities (PL III. fig. 13 ; PL IV. fig. 3). These occupy 

 the same relative position to one another and to the gastral limiting membrane of the 



