REPORT ON THE HEXACTINELLIDA. 193 



only suggested. The comparatively strong and straight basal rays, which are sparsely 

 covered with small externally directed teeth, are only about 0"05 mm. in lengtli. At 

 first gradually narrowing, they end in a short conical point. The distal ray, which has 

 an average length of 0'3 mm., is in some regions decidedly shorter, while it may, on the 

 other hand, attain double the above length. It is as a rule perfectly straight, strongly 

 developed at the base, and very gradually diminishing in diameter up to the fine narrow 

 point. It is covered with strong, distally directed spines or teeth, which stand out 

 somewhat markedly towards the base, and become shorter and more closely apposed 

 towards the extremity (PI. XXVII. fig. 12). 



In some positions on the lateral surface of the body the pinuli are specially long and 

 somewhat bent (PI. XXVII. fig. 11). The dermal skeleton of the superior terminal 

 sieve-plate does not differ in essential characters from that of the lateral wall. Instead 

 of the large pentact hypodermalia, however, strands of diacts of various sizes pre- 

 dominate. 



The marginalia forming the marginal fringe of the whole sieve-plate are straight or 

 but slightly curved oxydiacts, 1 to 1'5 mm. in length. Four distinct, cruciately disposed, 

 broad and rounded protuberances from the point of intersection represent the tangential 

 rays. The freely projecting distal portion bears externally directed spines, and 

 narrows very gradually to a fine point. The proximal portion, which is inserted in 

 the parenchyma, is either smooth or but sj^arsely beset with small pointed elevations, 

 which stand out transversely or are somewhat turned towards the apex, i.e., 

 internally. The proximal portion is always thicker and less pointed than the distal 

 external portion, and its length is to that of the latter generally in the proportion 

 of 1 : 2. 



Though it cannot be doubted that these marginalia belong to the rank of the 

 dermalia, and are most nearly related to the autodermal pinuli, no distinct transitional 

 forms are to be observed ; they can indeed be distiuguished with equal sharpness from 

 the pinuli of the sieve -plate and from those of the sides of the body. 



Special attention must be directed to those remarkable elements of the dermal 

 skeleton which were designated birotulate spicules by Bowerbank, and amphidiscs by 

 Max Schultze and others. They consist of a straight main stem, both ends of which 

 bear a similarly composed campanulate umbel formed of a varying number of radiate 

 processes, varying in length, and either of a leaf-like form, or narrow like the ribs of 

 an umbrella. This form of spicule is t}^ical and characteristic of the whole family of 

 the Hyalonematidse. Manifold variations occur in shape and size, and these are partially 

 characteristic of the different genera and species, and thus useful for diagnostic purposes. 

 That we have here really to deal with diacts is evident, in spite of the absence of any 

 intersection of the well- developed axial canal by one or two small transverse canals, from 

 the fact that amphidiscs are not unfrequently found, in which four, cruciately disposed, 



(ZOOL. CHALL. EXP. — PART Lm. — 1887.) Ggg 25 



