On Hexactinellid Spicules and their Names. M17 



XLV. — On Hexactinellid Spicules and their Names.—' 

 Part II. Supplementary. By R. KlRKPATEICK. 



(Published by permission of the Trustees of the British Museum.) 



In the : Annals ' for Feb. 1910 I sketched out a classification 

 of Hexactinellid sponge spicules on a morphological basis. 

 In the paper referred to it was stated that all Hexactinellid 

 spicules could be arranged in two groups, viz. Holactine 

 spicules without end-spines or distal appendages to their 

 actines, and Astral spicules with end-spines or distal append- 

 ages at the end of the actines. In the present paper attention 

 will be called to certain spicules which are undoubtedly 

 holactine, but which have spines at one end of the ray. In 

 spite of this fact these spicules do not belong to the astral 

 group, for the spines are not end-spines or distal appendages. 

 The spicules with these seemingly paradoxical characters are 

 the clavula?, scopulse, and certain root-tuft spicules such as 

 are found in Hyalonema. 



The scopula?, which are orientated more or less vertically 

 at the dermal and gastral surfaces of certain Dictyonine 

 sponges, are slender rods with two or more prongs projecting 

 from the outer or surface end of the spicule. The axial canal 

 extends the whole length of the rod, but the prongs are solid. 

 Under a high power and with good light it is not difficult to 

 see an axis cross — already figured by fSchulze — in the usually 

 swollen end whence the prongs emanate. Accordingly the 

 scopula is a micromonactin with five aborted actines, the 

 central or basal end of the rod being the end with the prongs. 

 The distal or apical end, which is situated in the interior of 

 the sponge, has no end-spines or distal appendages. Conse- 

 quently the scopula is holactine and not astral. From the 

 morphological point of view it is desirable to call appendages 

 situated at the central end of monactine spicules by some 

 distinctive name in contrast with such terms as "end-spine" 

 or " distal appendage, 3 ' and I suggest the designation 

 <{ centrospine." Similarly the disks of clavulse may be 

 termed centrodisks as compared with the distal disks of 

 amphidisks. For clavulse likewise are micromonactins in 

 which the axis-cross can be seen in the disk at the central 

 end of the spicule. Of course it may be urged that anything 

 beyond the point where the axial canal of the peisisting 

 actine joins the axial canals of the aborted actines should be 

 regarded as belonging to the territory of the latter, and that 

 the designation centrospine is not correct. The term is merely 



