140 



•EMDEAVOUR" SCIENTIFIC RESULTS. 



producing^ echinatingly-disposed basical megascleres. As 

 a final step the cells, which, by their proliferation, main- 

 tain the supply ol spongoblasts and scleroblasts, became 

 localised at the extremity of the papilla, the further growth of 

 which was thereby limited to increase in length. The manner 

 of growth of the fibre might accordingly be likened to that of 

 a Phanerogam stem — the spongoblast- and scleroblast-produc- 

 ing cells of the former corresponding to the apical meriste- 

 matic cells of the latter, and the spicules, like the leaves, 

 developing in acropetal succession. Thus at every stage of 

 growth the fibre would be surmounted at its extremity by a 

 "tuft" of newly-formed basical spicules, and it would depend 

 almost solely upon the initial orientation of these spicules with 

 regard to the direction of growth of the fibre, and their precise 

 location (whether at the extreme tip of the fibre or subtermin- 

 ally) whether they subsequently became wholly en\eloped by 

 the onwardly developing spongin as coring spicules or whether, 

 being more or less perpendicularly disposed, they were left 

 with only their bases imbedded in spongin, as echinafifif:; spi- 

 cules. When the fibre-forming spicules are of a single kind 

 the attempt to draw a distinction between coring and echi)iut- 

 ing spicules is, to a great extent, artificial, and usually breaks 

 down in practice ; as a matter of fact the spicules at the time of 

 their formation at the growing-point of the fibre are, in a 

 sense, all of them echinating. Accordingly Ophlitaspongia 

 and EchinocJathria which, by common agreement, were placed 

 in the "Ectyonina?," should logically have been included in the 

 " Dendoricinje. " 



The foregoing remarks concerning the mode of origin and 

 formation of basifugal fibres apply more particularly to those 

 of which the constituent spicules are basical megascleres only. 

 Although fibres of this kind are the rule, there are a number of 

 genera in which auxiliary spicules also participate in their 

 formation and some again in which these are the only fibre- 

 forming spicules. In these exceptional cases we may consider 

 either that the ascending sponginous fibres have come into 

 association with, and have enveloped in their progress the 

 "descending" (basipetal) strands of auxiliary spicules ;1 or that 

 in connection with the formation of the fibres, "auxiliary" 

 scleroblasts have come to take a regular place amongst the 

 cells of the fibre-growing point. From the point of view of 

 spicular constitution merely, the skeletal fibres of the Myxil- 

 linae are referable to three main types, according as basi(\Tl 

 megascleres only, basical and auxiliary both, or auxiliary 

 megascleres only, take part in their formation. 



1 In AulosvMigus tubulatus, Bowbk. (Proc. Zool. Soc, 1873, p. 29; Dendy, 

 Ann. Mag. Nat. Hist. (6), iii.. 1889, p. 29^. the fibres appear to grrow up in 

 an analagons way around the tubes of commensal worms. 



