42 SIDONOPS OXYASTIU. 



Below the cortex of the afferent areas numerous cavities, which appear 

 connected with each other by paratangential canals are met with (Plate 7, figs. 

 1, 2, 6). Into this system of subcortical cavities the radial afferent cortical 

 canals open out, and from it numerous narrow afferent canals, which extend 

 downwards into the choanosome, take their rise. 



The intermediate tissue is poorly tlevclopcd, the final ramifications of the 

 canals and the flagellate chambers being separated only by thin membranes 

 (Plate 6, fig. 3). The flag(>llatc chami)ers are, so far as I could make out, spheri- 

 cal and measure 17-25 /< in diameter. 



The efferent canals join to very wide (up to 1.5 mm.) efferent canal-stems 

 (Plate 7, figs. Id, 2d) which extend towards the efferent areas of the cortex, 

 below which they join to form a more or less continuous efferent subcortical 

 cavity. From this the radial efferent cortical canals take their rise. These 

 canals are 0.1-1 mm. wide, have a circular transverse section, and open out 

 freely on the surface. They are destitute of dermal pore-sieves (uniporal). 

 Their openings, the efferent pores (Plate 6, fig. 4; Plate 7, figs, le, 2e; Plate 8, 

 fig. 14), which occupy the efferent areas above described, are circular and, like 

 the canals which terminate in them, 0.1-1 mm. wide. The great difference in 

 size between the smallest and the largest of these jjores is remarkable. The 

 small ones are few in number and scattered irregularly among the much more 

 numerous large ones. The centres of the efferent pores are, irresjiective of the 

 width of the pores, quite uniformly 1.2 mm. apart, and the distance between 

 the margins of adjacent pores is consequently in inverse proportion to their size. 

 Tliis and the fact that the cortex is tliicker between large pores than between 

 small ones, seem to iinlicate that the great differences of width observed in the 

 efferent pores (cortical canals) are due to differences in degree of contraction. 



Skeleton. In the* interior of the choanosome numerous, rather irregularly 

 scattered amphioxes, some amphistrcmgyles, a few styles, large oxyasters, and 

 some stcrrasters, mostly young forms, are met with. Towards the surface 

 rhabds, similar to those mentioned above, together with the rhabdomes of 

 numerous subcortical plagiotriaenes and a few small subcortical anaclades, form 

 radial bundles which abut vertically or somewhat obliquely on the cortex (Plate 

 7, figs. 1, 2, 6). In this subcortical region of the choanosome and in the inner 

 layer of the cortex also minute dermal rhabds occur; the (young) sterrasters 

 are here much less abundant than in the interior, and the large oxyasters of the 

 latter for the most part replaced by large oxysphaerasters. In the middle layer 

 of the cortex the sterrasters form a dense mass. The dermal layer contains 



