OF THE SPONGIAD.E. 45 



that in the one the terminations of the haini are attenuated 

 and acute, and in the other they are expanded into concavo 

 convex discs. 



These two modes of development appear to be subject 

 to a considerable amount of variation in the growth of the 

 terminal discs ; as in some cases we find the distal part of 

 the terminal plate to consist of a uniform curve, while in 

 other cases the shaft is carried through the centre of that 

 curve, forming, as it were, a supplemental hook. These 

 variations are in perfect accordance with the general laws 

 of development in this class of spicula, as we find, both in 

 the'bihainate and anchorate forms, a considerable amount 

 of difference in the structure and position of these organs in 

 the same species of sponge. 



A similar organic relationship appears to exist between 

 the umbonate forms of bihamate spicula represented by 

 Figs. 115, 116 and 117, Plate V, and the eccentric 

 trirotulate forms represented by Figs. 133 and 134, 

 Plate X. 



2nd. Anchor ate Spicula. 



The anchorate spicula, unlike the bihamate forms, 

 appear never to occur reversed or contorted, but always 

 to present their terminations in the same position as those 

 of the bow of an ordinary ship's anchor. In some sponges 

 they are tolerably uniform in shape and proportions, while 

 in others they vary exceedingly, not only while in course of 

 development, but even in their adult condition ; they glide 

 so insensibly from one form into another that it is difficult 

 to draw a distinction between them ; and yet, notwithstand- 

 ing this latitude in shape and development, they are very 

 .characteristic of species, as there are always a sufficient 

 number of fully developed ones that exhibit the normal 

 form. 



In almost every case of their occurrence, beside the large 

 and fully developed organs, we find a secondary series 

 accompanying them, which are very much smaller in size, 

 and vary exceedingly both in symmetry and amount of 



