98 GEODIA MESOTRIAENA. 



protruding mesotriaenes, the inner layer also by the distal parts of the rhabd- 

 omes of those orthotriaenes and anatriaenes the cladomes of which lie near its 

 outer limit. The outer layer is 300-500 n thick. The middle layer, being chiefly 

 composed of sterrasters, firmly held together by connective fibres, appears as 

 a strong armour (sterraster-armour). Below the outer part of the surface this 

 layer is in all the three varieties 1-1.5 mm. thick, in the walls of the praeoscular 

 tubes thinner, in var. megana hardly half as thick (Plate 21, fig. 2). The tissue 

 composing the inner layer extends along some of the large, radial, choanosomal 

 canals a considerable distance into the interior; between these canals it is 

 usually 300-600 /i thick. 



As stated above the outer surface is covered with slight depressions 1.3-2 

 mm. apart. These are situated between the points where the radial spicule- 

 bundles abut on the surface (Plate 23, fig. 24), so that it appears as if these 

 depressions had been produced by a subsidence of the parts of the surface (cortex) 

 not supported by the radial spicule-bundles. The depressions are obviously 

 homologous to the depressed parts of the surface of horny sponges lying between 

 the conuh, while the elevated parts, supported by the radial spicule-bundles, 

 correspond to the conuli. Not only the outer surface but also the surface form- 

 ing the limit between the outer and middle layers of the cortex is raised in the 

 radii of the spicule-bundles, so that this also appears conulated; the "conuli" 

 of this limiting surface are even higher than those of the outer surface. The 

 surface forming the lower limit of the middle cortical layer is not thus raised in 

 the radii of the spicule-bundles and nearly continuous. The thickness of the 

 outer and middle layers of the cortex are consequently far from uniform; in the 

 depressions the outer layer is thicker and the middle layer (sterraster-armour) 

 thinner than in the radii of the spicule-bundles (under the conuli). The small 

 amphioxes in the outer layer of the cortex form radial tufts. Their outer ends 

 protrude some distance beyond the surface (Plate 23, fig. 25i; Plate 25, fig. lb); 

 on the outer surface of var. megana usually 50-80 n, in the praeoscular tubes of 

 this variety 200-300 Where the spicules penetrate it, the dermal membrane 

 is drawn up tent-fashion (Plate 25, fig. 1). Thus a great number of small, one 

 might say secondary, conuli are formed rising, everywhere from the surface 

 between the large (primary) conuli. 



The canal-system proper. In the depressions between the conuli, groups 

 of pores, penetrating the dermal membrane and rendering it sieve-like, are met 

 with. On the outer, exposed surface of var. megana these pores are more or less 

 circular and 40 60 /j. in diameter. They lead into canals (Plate 22, figs, lb, 2b; 



