HEXACTINELLIDS WITH DISCOCTASTERS. 45 



crook just mentioned is, in other cases, modified into a gentle bend and 

 in still others, completely straightened out so that now a principal and a 

 terminal jut out from the central node in a single straight ray. But 

 such a ray ought certainly not to be looked in the same light as the 

 primary undivided ray of a hexactin. The latter is invariably traversed 

 throughout its whole length by the axial canal, which, in the case of 

 hexasters, is likewise found in the principals but never extends into the 

 terminals. In conformity with the last mentioned fact, the apparently 

 simple and unforked hexaster rays already referred to, contain the axial 

 canal only at their bases, clearly demonstrating their constitution out of 

 a principal and of a single terminal. As is well known, one or more 

 rays in an oxy hexaster may be unforked or uniterminal ; and when all 

 are so and straight, as is of common occurrence, there arises a form 

 which is in shape a hexatin though not a genuine one in nature. Such 

 a spicule, when cleaned and examined in glycerine, will be found to con- 

 tain the usual central axial cross, the arms of which extend but for a 

 short distance into the bases of the six rays. The impropriety of simply 

 calling it oxyhexactin, as has hitherto been the custom (Schulze, Lambe, 

 Rauf), is evident. It should be called hexactin-shaped oxyhexaster. 



The microdiscohexasters are probably nevet: absent, although in 

 some species they occur quite sparingly. They are of usual shape and 

 vary in diameter from 15// to 20.6ft. 



The autodermalia are rough, straight diactins, stauractins, or pen- 

 tactins, one or the other of these predominating according to species. 

 Monactins, orthodiactins, and triactins are only of occasional occurrence. 

 As in all other Rossellids, a distally directed ray is never developed on 

 the autodermalia. When stauractins or pentactins constitute the main 

 elements, their cruciate rays are usually so arranged as to bring about 

 an autodermal lattice-work with more or less regularly quadrate meshes ; 

 whereas, in species with diactin autodermals, the meshes formed are 

 triangular, trapezoidal, or irregular in shape. 



A hypodermal system of spicules is always present. For Acanthas- 

 cus cactus it is characteristic that the hypodermalia consist exclusively 



