PLATE LXXIV. 211 



length of the skeleton ones, but their diameters are 

 not more than one seventh or one eighth of those of 

 the skeleton. The most characteristic of these 

 spicula is the tricurvato-acerate form. They are nor- 

 mally long and nearly straight, but they are frequently 

 more or less flexuous or distorted ; the middle curve 

 is mostly short and angular, like that of the printer's 

 bracket used to equalise two or more names. Some- 

 times the limbs of the middle angle are so distorted as 

 to assume the form of a round or oval loop. These 

 spicula are very slender, but they are often twice the 

 length of a skeleton spiculum. They occur in con- 

 siderable numbers in the dermal membranes, and are 

 most frequently grouped in loosely constructed fasciculi. 



The retentive spicula are beautiful, but very minute 

 objects ; they are rather long, with two equal sized 

 well developed and very symmetrical palms. They 

 are so small as to require a power of five or six 

 hundred linear to demonstrate their forms correctly. 

 They are dispersed rather sparingly on the membranes, 

 and are not readily detected in situ. 



The comparative measurements of these spicula 

 afford some very characteristic points for the dis- 

 crimination of the species. Thus, the diameter of 

 an adult skeleton spiculum is 13 1 63 inch ; and this 

 measurement is exactly that of the length of a full 

 sized pahnato-anchorate retentive spiculum. The tri- 

 curvato-acerate tension spicula are frequently nearly 

 twice the length of a skeleton spiculum, but while the 

 latter has a diameter of ysVs inch, that of the former 

 does not exceed the TWOO i ncn > or about ^ P art of 

 the stouter form. 



The interstitial membranes were abundantly supplied 

 with the dark amber-coloured sarcode of the sponge ; 

 but a close examination of them, aided by a strong 

 light displayed a sparing distribution on their surfaces 

 of each of the forms of spicula that have been de- 

 scribed as appertaining to the dermal membrane. 



