474 RICHARD EVANS. 



spicules already described, the main feature of these spicules 

 is the presence of a number of swellings, which varies from 

 one to five. As a rule they are situated symmetrically with 

 regard to the middle point of the spicule; that is, if there is 

 only one swelling, it is situated at that point, but if there are 

 two they are placed one on each side of that point, and at equal 

 distance from it ; and similarly the symmetry is retained when 

 there are three, four, or five swellings. The absence of the 

 symmetrical arrangement as seen in fig. 4, d, is very exceptional. 

 The axial thread, in contrast to spicules of class a, present a 

 dilatation corresponding to each swelling on the spicule. 



(y) The spicules in this class are of variable and irregular 

 form, since the individual amphioxea or amphitornota which 

 form them may fuse at any point, and at any angle (PI. 37, 

 fig. 3, a— y, especially fig. 2, d). As a rule, these compound 

 systems are formed from spicules of class a, though occa- 

 sionally a spicule of class j3 is found to take part in their 

 formation. 



With regard to their origin, two suppositions are possible; 

 first, that they are the result of irregular growth, and branching 

 of a single spicule derived entirely from a single scleroblast ; 

 secondly, that they arise by fusion of spicules primitively 

 distinct, and formed each by its own scleroblast. Fig. 3, e—j, 

 might be taken as evidence for the former view, but such 

 forms as that represented in fig. 3, d, render such a supposition 

 highly improbable, to say the least. The view that these 

 spicular systems are of compound origin receives strong 

 support from the way in which their axial threads cross one 

 another instead of branching. If these irregularities arose as 

 outgrowths from one spicule formed in one mother cell, it 

 might well be expected that their axial threads should be also 

 formed as outgrowths from that of the main spicule, and would 

 therefore be continuous throughout the system, but this is 

 certainly not the case in many spicules of our Spongilla, as can 

 be seen from the figures. In another sponge, which is probably 

 a monaxonid of the family Axinellidas, viz. Tricentrium 

 muricatum (Pallas, 1756), Ehlers, 1870 ( = Plectronella 



