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



hanging expansion of the curved outer end of the ray. In the former case they are 

 radially symmetrical, in the latter bilaterally symmetrical, with a plane of symmetry, 

 indicated by the S-shaped curvature of the terminal ray. 



Although the radial terminal discs vary greatly in form, they all agree in this that 

 the outer terminal surface is convexly arched or conical, while the lateral margin is 

 provided with small teeth or longer protuberances. If the marginal teeth are large in 

 relation to the central body of the disc a many toothed anchor form results (PL XXV. 

 fig. 6). If the middle portion of the anchor, on the other hand, is more strongly 

 developed, the result is a hemispherical form with a toothed margin, or a campanulate form 

 with long marginal protuberances (PI. XII. fig. 4). The number and form of the marginal 

 teeth varies in different rosettes. It is frequently four, but in other cases six, eight, 

 twelve, or indefinitely more. 



To all rosettes in which the secondary rays bear on their ends a transversely directed, 

 radially symmetrical terminal disc, I would apply the term " discohexasters." Those 

 forms, however, in which S-shaped terminal rays bear on their ends a bilaterally 

 symmetrical disc-like thickening, I would call "Jloricomes " — a term used in reference to 

 the resemblance of the bundle of rays to a flower perianth, and first employed by 

 Bowerbank in the description of Euj^flecteUa aspergillum. In these floricomes the 

 formation of small marginal teeth is usually confined to the terminal portion of the disc- 

 like thickening that projects or overhangs externally (PL III. fig. 11; PL XIX. fig. 5), 

 so that a hand or claw-like form arises. In other instances smaU teeth are formed over 

 the whole margin of the disc-like thickening (PL XVIII. figs. 6, 7). 



With these regular hexacts numerous irregular forms are contrasted. The latter are 

 characterised by the variable formation of one or more rays ; the distinctions are 

 associated with difference in length of ray, or with modification of form. To the first 

 category belong the sword-like hexacts, which are found beneath the skin of many 

 Eujjlectellidse, such as Euplectella aspergillum, (PL III. fig. 10), Euplectella crassistellata 

 (PI XII. fig. 4), and others. In these the five outer rays are simple, smooth, round, and 

 run out to a point ; they are either altogether or almost exactly of equal length, while 

 the sixth ray, directed inwards, is also smooth, round, and pointed, but is fully twice as 

 long as the others. Frequently, too, besides the ray directed inwards, the outer radius 

 on the same axis is longer than the tangentials, so that the resemblance to a cross-hilted 

 sword may become very obvious, as in Tasgeria pulchra (PL XL fig. 5). This lengthen- 

 ing of the rays, in one or in two axes, is quite frequent, e.g., in those hexacts with 

 long thread-like rays, which are found so abundantly in the parenchyma of Malac- 

 cosaccus vastus. In this form the two rays on the radial axis are usually shorter than 

 the four tangentials. The inverse relation occurs in the hexacts which Lie in the gastral 

 membrane of Asconema setuhalense, where the two rays of the radial axis are longer than 

 the four tangentials (PL XXI. fig. 5). 



