GEODIA MESOTRIAENA. 



101 



cells and a coating of densely packed euasters intervening between the layer of 

 circular fibres and its lumen. The transverse membranes, crossing the radial 

 canals, are composed of three layers, an upper and a lower superficial granular 

 layer, and a central transparent, and more highly refractive, apparently fibrous 

 layer about 10 n thick. 



In the tissue free from flagellate chambers which as stated above, envelops 

 some of the radial canals, large elongate cells are here and there met with singly 

 and in groups. These cells (Plate 22, fig. 9a, b) appear to be destitute of a 

 membrane; their protoplasm is coarsely granular; their nucleus oval or spherical. 

 Most of these cells are thick spindles drawn out at each end to a point (Plate 

 22, fig. 9a); some of them, however, are thus drawn out at one end only and 

 rounded off at the other, so that they appear pear shaped (Plate 22, fig. 9b). 

 They attain a length of 30-50 and a breadth of about 12 ji. The nucleus meas- 

 ures 4 in diameter. In some of them I have observed small masses of easily 

 stainable granules close to the nucleus. In others, areas, more transparent and 

 less stained than the other parts, are met with in the protoplasm. These cells 

 seem to be ova. 



Skeleton. In the innermost parts of the choanosome, that is, those farthest 

 from the outer surface and the praeoscular tubes, masses of large amphioxes 

 and a few large styles are found. Some of these are spicules irregularly 

 scattered; the majority join to form loose strands extending towards the elevated 

 parts. Bundles of spicules arise from these central masses (strands) and extend 

 radially towards the surface. In the smaller and more solid specimens of var. 

 microana and var. megana most of these bundles are straight or only slightly 

 curved and abut vertically on the surface (Plate 21, fig. 2). In the larger speci- 

 men of var. pacJiana, the structure of which is more complicated, only the bundles 

 extending towards the summits of the elevations (gyri) are straight and in their 

 distal parts vertical to the surface, while the others curve strongly, attempting 

 as it were, to reach the flanks of the gyri vertically, which, however, they do not 

 succeed in, so that their distal ends abut obliquely on the surface. Like the 

 central spicule-masses (strands), the proximal parts of these radial bundles are 

 composed entirely of large rhabds (numerous amphioxes and few styles). In 

 the distal parts of the radial bundles also rhabdomes of telo- and meso-clades 

 occur. The teloclades are mostly anatriaenes and ortho- or plagio-triaenes, the 

 mesoclades mesoprotriaenes. Besides these normal forms triaene-derivates with 

 two clades or only one occasionally occur. The rhabds (amphioxes and styles) 

 extend to the distal part of the choanosome, but usually do not reach the cortex. 



