Skeleton of Euplectella aspergillum, 47 



which, however, are usually produced, the other arms being 

 more or less suppressed or abortive ; the tips of the anus 

 terminate in a kind of spear-head form which is spined; 

 these arms are all extremely long, and some exceedingly 

 slender, and they are twisted and bent in a variety of ways 

 to suit the pattern of the network. 



The open network of the mouth of the tube is composed 

 of similar spicules interwoven together. 



The exterior " ridges or frills " are also built up of spicules 

 of the same form, the arrangement being tliat two or more 

 long arms or rays lie along the network of the skeleton, 

 whilst another arm, which is a short or dwarf one, stands up 

 perpendicularly to the surface. These short upright arms are 

 so placed as to form the lines of the ridges ; and amongst them 

 are interlaced the long slender rays of other spicules, in some 

 of which all the transverse arms are suppressed. These ridges 

 therefore contain the principle availed of in mechanical engi- 

 neering in the J. iron girder. 



Interspersed throughout the whole of the network are nume- 

 rous smaller sexradiate spicules, chiefly of slender form ; but 

 there are some with short stout arms which are quite smooth • 

 the arms of the slender ones are all spined at the ends, and one 

 of the longitudinal arms is usually produced to a great length. 



The flesh-spicules are the "rosettes" of the two kinds de- 

 .scribed and figured by Mr. H. J. Carter in the November 

 Number of 'Ann. and Mag. of Nat. Hist.' plate xiii. figs. 4 

 and 11, 1873. 



Exteriorly there are bundles of long straight spicules or 

 rods running along the longitudinal lines of the skeleton 

 for some distance from the base, some of which are smooth, and 

 others barbed. 



It is a matter of great satisfaction to have met with a 

 specimen of this species of sponge in which the vitrification 

 is in progress : here it is actually so. In the lower part of the 

 network to which the vitreous cement has been applied, it 

 has been deposited in sufficient quantity to render the skeleton 

 rigid, but not in such quantity or so completely as in the usual 

 rigid examples. The examination of a portion, after boiling in 

 nitric acid and subjecting to considerable heat, shows that the 

 silicious coating has been deposited in layers, just as in the 

 case of the spicules, the only difference, apparently, being 

 that as cement it is amorphous, whilst in the spicules it 

 assumes definite forms. The spicules of this species are, 

 as Mr. Carter remarks, imbedded in vitreous material, much 

 in the same manner as in the horny-fibred sponges the spi- 

 cules arc imbedded in horny material. In the Chalinai, Jiow- 



