48 SIDONOPS BICOLOR. 



0.5-1.5 mm. in diameter. The centres of these pore-groups are 1.5-2.5 mm. 

 apart, ^^'hen the groups are large and their centres close together, as is the case 

 on parts of the surface of one of the specimens from Station 4531, the pore- 

 groups come in contact with each other and form a fairly continuous sieve. 

 When however, as is more frequently the case, the pore-groups are smaller and 

 farther apart, they appear divided by a network of belts free from pores. The 

 pores themselves are oval, measure 100-300 /( in diameter, and perforate the 

 thin dermal membranes covering the distal widened parts of the afferent canals 

 (Plate 9, fig. 16), which traverse the cortex in a radial direction. These canals 

 are cylindrical in the centre, and up to 400 /( wide. They widen above in a funnel- 

 shaped manner, and are contracted below by a stout chonal sj^hincter, which 

 hes at the level of the limit between cortex and choanosome. They lead into 

 subcortical cavities of no great size which lie just below the cortex and from which 

 the afferent choanosomal canals take their origin. The flagellate cliambers 

 (Plate 9, fig. 18) are spherical and 20-32 ji in diameter. The efferent canals are 

 l^rovided with sjihincter-mcmbranes at frequent intervals and join to form tubes, 

 often as much as 1-1.6 mm. in diameter, which lead up to the efferent areas of 

 the cortex (Plate 9, fig. 17). Some appear to end at the limit between cortex 

 and choanosome, while others bend round and continue their course paratangen- 

 tially for some distance just below this level, thus forming efferent subcortical 

 cavities. From the ends of the former and the roofs of the latter the efferent 

 cortical canals arise. These are constricted at their origin by chonal sphincters 

 lying at the level of the limit between cortex and choanosome. Beyond the 

 sphincter the canal widens to a cylindrical tube 250 ju-1 mm. in diameter, which 

 traverses the cortex radially (Plate 9, fig. 15) and opens out freely on the surface 

 (Plate 10, fig. 14). The efferent pores, in which these canals terminate, either 

 have nearly the same width as the canals themselves or they are slightly 

 smaller. The centres of these efferent pores are 1-2.5 mm. apart, their dis- 

 tance being on the whole proportional to their size, small ones lying much 

 clo.ser together tlian large ones. These efferents, though usually restricted to 

 concave parts of the surface which are generally free from afferent pores, are 

 also found on other parts of the surface, irregularly distributed between the 

 groups of afferents. On large parts of the surface no pores of any kind can be 

 made out. 



Skeleton. Spicule-bundles which widen out distally traverse the choano- 

 some (Plate 9, fig. 17) radially and abut vertically or somewhat obliquely on 

 the surface. These bundles consist chiefly of amphioxes, of which two kinds, 



