142 "ENDEAVOUR" SCIENTIFIC EESULTS. 



(\ii.) Principal and accessory spicules are more or less 

 well disting-uished ; the former echinate the fibres, 

 the latter are interstitial and dermal. C reiki. 



(viii.) The "basicals" are scarcely- or not at all differ- 

 entiated into two kinds or are represented onl}- hv 

 accessory spicules. Clathrissa, Plumohalichondria , 

 Styloteilopsis, ll'ilsonella, Fusifer, ''Echiuo- 

 dictyutii'' spongiosum,'^ "£." arenosum,'^ "'Micro- 

 ciofHi" scabiila (p. 150). 



(ix.) The "basicals" are represented only by principal 

 spicules. EcJiiiiochalina. 



III. — The fibre-forming spicules are "auxiliaries" only; 

 basical spicules, if present, are (so far as known) of a single 

 kind, and, on account of their spination, appear to belong- 

 rather to the category of accessory than of principal mega- 

 scleres. 



(x.) Basical megascleres are present. Psi'iicJoclathria 

 Grayc'Ua, Hisiodcrmella, Microtylotella. 



(xi.) Auxiliary megascleres only are present. Species 

 ot lofrocJiota and MelonancJiora, and certain 

 species included in the Mycalinte. 



In the case of those Myxillin^e in which the megascleres are 

 of three kinds, the homologies of the spicules are, as a rule, 

 obvious ; and almost invariably it is found that the directive or 

 axial spicules of the fibres belong to the principal, the echinat- 

 ing spicules to the accessory, and the interstitial or dermal to 

 the auxiliary category of megascleres. \\'hen this mode of 

 arrangement of the spicules obtains, or when it is departed 

 from only through the loss of accessory spicules, the skeleton 

 might be described as being of the normal type, since it is that 

 in particular which is characteristic of most Myxillinae. 

 Amongst "trimegascleric" genera, Crella is exceptional in the 

 fact that in it the fibres are cored by auxiliary and echinated by 

 principal megascleres whilst the accessory occur extra-fibrallv- 

 From a study of the different forms assumed by the spicules 

 (both megascleres and microscleres) amongst the normal Myx- 

 illina?, we obtain much information which is of service in 

 enabling us to form a conclusion concerning the identity of the 

 megascleres in cases where the skeleton is of an anomalous or 

 aberrant type. J'hus there can be absolutely no doubt that the 

 "skeletal" spicules of Melonanchora emphysema, Forcepia 



1 Dendy— Proc. Eoy. Soe. Vict., viii., 1896, p. 50. 



