GEODIA ACANTHTYLASTRA. 



189 



Skeleton. In the inner proximal part of the choanosome rather irregularly 

 disposed amphioxes, sterrasters, and acanthtylasters occur. In the distal part 

 of the choanosome and the inner layer of the cortex similar amphioxes and the 

 rhabdomes of plagio-, mcsopro- and ana-triaenes form loose radial bundles. 

 The cladomes of the plagiotriaenes and of many of the anatriaenes lie just below 

 the sterraster-armour. Most of the mesoprotriaenes observed penetrate the 

 cortex, their cladomes protruding freely beyond the surface (Plate 46, fig. 20d). 

 Minute dermal rliabds, mostly disposed radially, small strongylosphaerasters, 

 and large oxysphaerasters are also found in this region besides acanthtylasters 

 and sterrasters similar to those of the interior. The sterrasters are not nearly 

 so abundant here as in the proximal part of the choanosome (Plate 46, fig. 20). 

 The sterraster-armour layer is occupied by sterrasters not very closely packed, 

 a few small strongylosphaerasters, radial minute dermal rhabds, and the parts 

 of the rhabdomes of the mesoprotriaenes which penetrate the cortex. In the 

 walls of the radial cortical canals, which traverse this layer, large oxysphaeras- 

 ters are met with. The dermal layer is occupied by groups of more or less 

 radially disposed, minute dermal rhabds, which are nearly all amphiox. The 

 proximal ends of these spicules are deeply implanted in the sterraster-armour; 

 their distal ends protrude freely beyond the surface (Plate 45, fig. 39c; Plate 46, 

 fig. 20a). The superficial part of the dermal layer, that is to say, the dermal 

 membrane, is occupied by a thin but dense layer of small strongylosphaerasters. 

 Similar spicules are also found in small numbers in the walls of the dermal canals. 



The spicules to which the following descriptions refer were taken from the 

 largest specimen. 



The choanosomal amphioxes (Plate 45, figs. 17-19a) are slightly curved in a 

 simple or irregular manner, 0.7-2.2 mm. long, and 14-40 pt thick. A smaller 

 and a larger kind of amphiox, which are connected by relatively few transi- 

 tions can be distinguished. The smaller ones measure 0.7-1.4 by 14-25 /_/, the 

 larger 1.5-2.2 mm. by 23-40 fi. 



The minute dermal rhabds (Plate 45, figs. 20-22, 39c) are nearly all amphi- 

 oxes. Styles have also been observed among them, but they are exceedingly 

 rare. The minute dermal amphioxes are 150-300 /t long and 3-15 pt, usually 

 8-13 n thick. The thickness is usually proportional to the length. Those over 

 265 jj. long are 12-15 /i thick, while those less than 200 fi long are less than 7.5 /.i 

 thick. Among those intermediate in length both thick and thin ones were 

 observed. Styles, although, as mentioned, very rare, were observed both 

 among the thick and the thin rhabds. They are about 250 /j. long and 4-10 // 

 thick. 



