182 GEODIA LOPHOTRIAENA. 



canals are surrounded by mantles of tissue, about 20-30 /( tliick, wliicli are free 

 from sterrastcrs. 



Skeleton. In the internal (basal) part of the ehoanosome loose, irregular 

 strands of aniphioxes occur. These mostly extend more or less paratangentially. 

 In this region also large, smooth oxyasters and smaller, spined sphaerasters, 

 mostly strongylosphaerasters, are met with. The distal, sulx-ortical part of 

 the choanosome is traversed by radial buniUes composed of amphioxes and the 

 rhabdomes of plagiotriaenes, dichotriaenes, other lophotriaenes, anatriaenes, 

 and mesoprotriaenes. Asters, similar to those of the interior, and minute, 

 radially arranged, mostly amphiox-rhal)ds also occur in this region. The 

 sterraster-armour is occupied by not very closely packed sterrasters lying from 

 three to five deep. In the dermal layer large masses of sphaerasters, which are 

 particularh- densely packed just below the surface, in the dermal membrane, 

 are met with. More or less radially arranged minute dermal rhabds, for the 

 most part amphioxes, are implanted in the cortex. Many of these spicules 

 travei-se the whole of it, their proximal ends being imbedded in the distal part 

 of the choanosome and their distal ends protiiiding freely beyond the surface. 

 A few minute dermal anatriaenes also occur in tliis region. The cladomes of 

 the plagiotriaenes, the dichotriaenes, and the other lojihotriaenes lie just below 

 the sterraster-armour (Plate 47, fig. 23). At a slightly lower level a good many 

 anatriaene-cladomes and a few mesoprotriaene-cladomes are met with. The 

 relative frequency of the plagiotriaenes and the dichotriaenes (lophotriaenes) 

 is different in different specimens. In some the simple plagiotriaenes, in others 

 the dichotriaenes (lophotriaenes), appear to form the majority. 



Most of the large choanosomal amphioxes (Plate 48, figs. 3-6) are quite stout, 

 1.2-1.8 mm. long and 25-42 fi tliick. The great majority of those thinner than 

 25 fi arc shorter than 1.2 nma. and appear as young forms of the stout amphioxes. 

 Occasionally, however, long and very thin amphioxes (Plate 48, fig. 7) are met 

 with, which can hardly be considered as young stages of the stout ones. One 

 of these slender spicules was 2 mm. long and only 10 p. thick. 



Most of the minute dermal rhabds (Plate 48, figs. 8, 9, 10a) are rather blunt 

 pointed, fairly isoactine amphioxes, but anisoactine forms, with one end more 

 slender and more sharjDly pointed than the other, also occur. In some of these 

 spicules (Plate 48, fig. 10a) the axial thread (canal) is greatly widened towards 

 the thicker end. The minute dermal rhabds are 110-200 fi long and usually 

 3-6 fi thick. 



The plagiotriaenes (Plate 47, figs. 17, 21) usually have a conical rhabdome 



