240 ANATOMY AND PHYSIOLOGY 



Fig. 



77. lusiFORMi-PORRECTO-TERNATE, a Very early stage of 



development, from Tethea cranium, Johnston, x 

 660 linear. 



78. A further stage of development of a fusifcami-porrecto- 



temate spiculum from Tethea cranium, Johnston. 

 X 260 linear. 



79. An adult fusiformi-porrecto-temate spiculum from 



Tethea cranium, Johnston. X 160 linear. 



80. A fusiformi-porrecto-temate spiculum from Tethea 



cranium, Johnston, charred to exhibit the cavities 

 of the shaft and radii. X 260, linear. 

 These spicula form the greatest portion of the fasci- 

 culi of defensive spicula virith which the external surface 

 of Tethea cranium is armed. They are very long and 

 slender, frequently exceeding a quarter of a inch in 

 length, with a diameter of -^^^ of an inch at the 

 thickest portion of the shaft. The ternate radii are 

 projected from the apex of the shaft at about an angle 

 of 20° from its axis, and are about -^^ of an inch in 

 length. See Fig. 362, a, Plate XXXI. 



81. EusiFORMi-RECDKVo-TERNATE spiculum, in au early 



stage of development, from Tethea cranium, John- 

 ston. X 260 linear. Page 32. 



82. PusiFORMi-RBCURVo-TERNATE, au adult spiculum from 



the same sponge as the spiculum represented by 



Mg. 81. X 260 linear. Page 32. 



This form of defensive spiculum occasionally ac- 

 companies the porrecto-ternate ones of the defensive 

 fasciculi of Tethea cranium. The length and propor- 

 tions of the shaft of the former are very much the same 

 as those of the latter. The recurvate apex of the 

 spiculum undergoes a progressive development, which 

 does not appear to commence until after a great 

 extent of the length of the slender flexible shaft has 

 been produced, when an enlargement of the apex of 

 the shaft takes place, and the rudiments of the stout 

 recurvate radii appear as represented by Pig. 81, and 

 between this and the fully-developed form, fig. 82, all 



