REPORT ON THE HEXACTINELLIDA. 65 



however, is here and there disturbed by the iuterpolatiou of separate gaps, and it 

 ceases immediately beneath the superior cuff, in the formation of a perfectly circular 

 row. 



The distance between two adjoining parietal apertures in the same row is, in the 

 upper portions of fully-developed specimens, about 4 mm.; at the lower end, on the other 

 hand, and in young forms the distance is only 3 mm. or less. In young specimens, and 

 on the inferior portions of older forms, a slightly arched protuberance is formed in the 

 middle of every four adjacent gaps. By the fusion of the elevations which lie between 

 two adjacent spiral rows, raised bands are formed, which, by further increase, become 

 prominent ledges or ridges. The arrangement and development of these ridges vary 

 greatly on the walls of the lower part of the tube, they are always but feebly developed, 

 but they gradually increase in height towards the upper end. In some specimens they 

 hardly attain a height of 2 or 3 mm.; in other cases, however, they rise to a height of 

 10 mm. or more. Though they run, for the most part, parallel to the spiral rows of gaps, 

 they seldom continue in the same direction, beyond a semirevolution. They often bend 

 round at right angles, just at the end of half a spiral turn, and extend in the opposite 

 direction down the other side of the tube. Sometimes again they assume an angular or 

 undulating course, and here and there they may even form a meshwork. In specimens 

 bent in an S-like curve, I usually observed several parallel ridges extending in an 

 ascending spiral on both sides, from the lower concavity of the tube to the upper 

 concavity on the opposite side (PL I. fig. 1). 



Among the ridge-like formations must also be ranked that ring-like smooth ledge 

 which borders the terminal sieve-plate, and which, though in many cases merely 

 suggested, attains in others a height of 10 mm. Usually, however, the "cuff" is not 

 in direct connection with the rest of the ridge system, but is separated from it by a 

 concave circular zone varying from 5 to 10 mm. in breadth. It is distinguished from the 

 other ridges by being thinner, and by possessing sharper edges. 



On the inner surface of the wall of the tube may be observed a tolerably regular 

 system of circular and longitudinal ledges, which together form a quadrate lattice-work. 

 The circular ledges, however, extend somewhat further inwards than the longitudinal. 

 The quadrate meshes which are thus formed have a breadth of 3 or 4 mm. They difler 

 essentially from one another, inasmuch as the one series exhibit in their centre 

 the circular parietal gaps already mentioned, with several minute grooves at the 

 circumference, while the others only show one or more furrow-like grooves of various 

 breadth and depth. The areas which exhibit deep furrows correspond to those ridges 

 which are prominent externally, while the shallower grooves underlie the simple 

 elevations that occur between every four gaps, in those regions which are not provided 

 with ridges. The arrangement of the parietal apertures in oblique spkal rows, and the 

 rhombic form of the areas occurring between every four adjoining gaps, are conditioned 



(ZOOL. CHALL. EXP. — PART LIII. — 1886.) Grgg 9 



