28 



SIDONOPS ANGULATA. 



and are not numerous. The oxysphaerasters are very abundant, particularly 

 in the inner layer of the cortex, where they line the walls of the subcortical 

 cavities in large numbers (Plate 14, fig. 22c). These asters also occur in the 

 walls of the chonal canals and they extend distally along the walls of the dermal 

 cavities, right up to within a short distance of the outer surface. The strongylo- 

 sphaerasters occupy the dermal membrane (Plate 13, figs. 22-24; Plate 14, 

 fig. 20a) in a single or in several layers and are met with also in the walls of the 

 cortical canals and subcortical cavities. On the whole the strongylosphaerasters 

 situated superficially appear to be larger than those in the interior of the cortex. 

 The sterrasters form dense masses in the middle armour layer of the cortex and 

 occur scattered also in the choanosome, where they are particularly abundant 

 in the lobose specimen of var. megana and in var. microana. 



Besides these microscleres, I found, in the centrifugal spicule-preparations 

 of var. microana, two oxy asters with more slender, spined rays, and in those of 

 the lobose specimen of var. megana numerous minute rhabds. Both these kinds 

 of spicules I consider as foreign. 



The stout choanosomal amphioxes (Plate 12, fig. 17c; Plate 13, figs. 1, 2, 17a, 

 b, 20b) are fairly isoactine and quite sharply pointed. They are usually straight 

 (Plate 13, fig. 2, 17b) or slightly curved (Plate 13, fig. 1), more rarely angularly 

 bent (Plate 13, fig. 17a) near the middle. The angular bend amounts to 8-12°, 

 so that the two actines of these angular amphioxes enclose angles of 168-172°. 

 These angularly bent choanosomal amphioxes are more frequent in the lobose 

 specimen of var. megana and in var. microana than in the other two specimens. 



The stout choanosomal amphioxes are 1.6-3.7 mm. long and 20-72 /< thick. 

 Their thickness is on the whole proportional to their length. They are con- 

 siderably longer in var. megana than in the other two varieties. 



DIMENSIONS OF STOUT CHOANOSOMAL AMPHIOXES OF SIDONOPS ANGULATA 





Length, mm. 



Maximum thickness, |x 



Var. megana, lobose specimen 



2-3.7 



40-60 



Var. megana, massive specimen 



2.4-3.5 



53-72 



Var. microana 



1.8-2.8 



20-52 



Var. orthotriaena 



1.6-2.5 



20-70 



The slender dermal amphioxes (Plate 12, figs. 16a, b, 17a; Plate 13, fig. 21b) 

 are usually simply curved (Plate 12, figs. 16a, 17a), rarely angularly bent (Plate 



