THE president's ADDRESS. 233 



shaft, expanded at the lower end to form a knob-like base — 

 which I propose to call the manubrium — ornamented with two 

 circles of short spines. For some distance above the manu- 

 brium the shaft is free from outgrowths, but a little beyond the 

 middle of the spicule there comes a whorl of three flattened 

 lobes with denticulate or crenate margins, followed after a short 

 interval by a similar whorl, and then, after another short in- 

 terval, by a third whorl which differs from the other two in not 

 being subdivided into three lobes. Following immediately on 

 the third whorl is the broad, hemispherical apex, provided with 

 a number of short, capitate spines and forming a sort of crown. 



As we shall have to refer to them frequently and individually 

 in what follows, it is necessary to distinguish the three whorls 

 by special names, and for reasons connected with their develop- 

 ment I propose to call them, in the order in which I have just 

 described them, the median, the subsidiary and the apical 

 whorls respectively (fig. 7, m.w., s.w. and a.w.). 



In Latrunculia apicalis (figs. 20-24) there is a very interesting 

 specific difference in the form of the discorhabd, for, beyond the 

 apical whorl, the spicule, instead of terminating in a crown of 

 capitate spines, is produced into a very long, slender, tapering 

 process resembling the " leader " of a growing fir-tree, to which, 

 indeed, the spicule bears a striking resemblance. We may call 

 this the apical prolongation {a.p.). Near the base of this process 

 there is a single or double circlet of small spines, which may 

 perhaps be regarded as a second subsidiary whorl (s.w. 2). An- 

 other difference, which is of considerable interest in connection 

 with the vibratory theory, lies in the fact that the median whorl 

 is a little farther from the apical whorl than in L. bocagei, being 

 approximately midway between the apical whorl and the 

 manubrium. The length of the fully grown spicule is about 

 0-126 mm.* 



Of course individual variations in the form of the discorhabd 



* Kirkpatrick, in his Report on the Tetraxonida of the National 

 Antarctic Expedition, has described two supposed varieties of L. 

 apicalis, in one of which two forms of discorhabd occur, one with and 

 the other without an apical prolongation, I doubt whether either 

 variety should be assigned to L. apicalis. 



