NARRATIVE OF THE CRUISE. 



4413 



" As one of the most important results of the carrying out of these detailed investi- 

 gations, the fact lias been established that the Hexactinellida, which were first clearly 

 marked off and characterised by Oscar Schmidt in 1870, form a division of the siliceous 

 sponges, definitely bounded on all sides, whose members are intimately united by a 

 common plan of structure. The subclass Hexactinellida 

 is, however, principally characterised by the triaxial or 

 six-rayed type, which underlies the forms of its spicules, 

 and also by the close agreement of the organisation 

 of its soft parts. In no single instance was I ever in 

 doubt whether 1 had before me a Hexactinellid or not ; 

 for even when many isolated spicules and the several 

 parts of a connected trabecular skeleton did not show 

 the typical Hexactinellid structure without further inves- 

 tigation, yet on careful examination this could be 

 demonstrated, and spicules were found showing either 

 the usual six-rayed form or an easily recognisable deri- 

 vative from it. As Oscar Schmidt was the first to point 

 out, the determination of the axial-relations of the 

 central canal is of special importance ; by means of study- 

 ing it in every connected trabecular skeleton the indivi- 

 dual six-rayed spicules, already partially united, are 

 always easily recognised ; even in the case of many 

 highly modified isolated needles, the central canal gives 

 a clue to the derivation from the typical six-rayed form. 

 However great the number of forms of the spicules in 

 the Hexactinellida may be, yet there are fundamentally 

 but few principles of modification which have been carried 

 out. These are — (1) unequal elongation of the individual 

 rays, in which may be found all degrees of shortening, 

 even to complete atrophy of one or more rays ; (2) divi- 

 sion of the rays into two or more branches ; (3) flexion 

 of the rays or their branches ; (4) unequal thickening of 

 the rays or their branches, which may lead to the develop- 

 ment of swellings of various forms, hooks, teeth, or the not infrequent terminal knobs 

 or toothed plates. 



" As in the case of the skeleton, so also in the general structure of the soft parts, 

 a predominant principle might be recognised. In all Hexactinellids, that surface (usually 

 the outer) which serves for the ingress of water, is covered by a thin perforated skin or 

 membrane (which is supported by a special system of regularly arranged spicules), accord- 



Fio. 165. — Lefroyella deevra, Wyv. Thorns, 

 (natural size), a representative of the 

 Euretiibv. 



