REPORT ON THE HEXACTINELLIDA. 33 



The shortening of one of the six otherwise similar rays is very frequent, and leads 

 finally to the formation of pentacts. 



Much more frequent, however, than mere differences in length, are modifications of 

 form. A ray may, in its entire configuration, more or less markedly differ from its five 

 neighbours. This is the case, e.g., in many hexacts with fir-tree-like distal rays, pinuli, 

 such as occur in the skin oi Aulascus johnstoni (PI. XXH. fig. 3), Caulophacus elegans 

 (PL XXV. figs. 4, 5), and many other Asconematida^, and also in Aphrocallistes 

 (PI. LXXXIV. fig. 8 ; PL LXXXV. fig. 4). More frequently, however, the diflferentia- 

 tion affects two radial rays generally on one and the same axis, as is usually the case in 

 the hexacts which occur in the skin of the Asconematidse, and are provided with 

 one prominent, scaly, fir-tree-like, pronged ray (PL XXII. fig. 9 ; PL XXVI. fig. 9). 

 Similar forms occur also in many EuplecteUidse (PL XII. fig. 3 ; PL XIII. fig. 2). 

 Apart from the differences already noted, in regard to the rays of many rosettes, 

 some other hexacts, with secondary rays, exhilait further irregularities. Thus, e.g., 

 Aphrocallistes heatrix. Gray, is markedly distinguished by the character of the spicules 

 which lie separately in the soft parts. In these, two of the principal rays, not however 

 on the same axis, are elongated and curved, and occasionally divided into four pointed 

 terminals, while the four other principals remain simple and short (PL LXXXIV. 

 figs. 9, 10). 



Pentacts. 



In support of the theory that pentacts have phylogenetically arisen from hexacts by 

 the atrophy of one ray, the existence of numberless transitional forms, with a more or 

 less manifest rudiment of the sixth ray, may be adduced ; while it ought to be noted 

 that the disposition of the five well-developed rays is in thorough agreement with 

 that of the corresponding rays in hexacts and, finally, that pentacts occur almost 

 exclusively close to the bounding layer, where the development of one of the two rays 

 standing at right angles to the limiting surface, is either impeded or specially favoured. 

 Either the distal or the proximal ray may thus undergo atrophy, with the associated 

 increase of the other. 



The rays may be smooth (PL LVI. fig. 6), or rough (PL LV. fig. 3), cylindrical or 

 attenuated tovrards the exterior, and pointed, rounded, or even thickened at the 

 extremity. 



The simple case in which all the five rays are of ec^ual form and size is of frequent 

 occurrence (PL LV. fig. 3), but the unpaii-ed ray usually difiers in some point from the 

 four others. It may project freely beyond the bounding surface, bearing lateral 

 prickles which overlap one another like scales. The resemblance to a fir-tree, thus pro- 

 duced, has earned for the entire spicule the designation pinulus. Such pinuli, in which 



(ZOOL. CHALL. EXP. — PART LHI. 1886.) Ggg 5 



