26 



SIDONOPS ANGULATA. 



fig. 4a), were observed. Each of these cells contains a small, strongly staining 

 nucleus. Perhaps these cells are young spermatozoa. 



Canal-system. The sides of the massive specimen of var. megana are cov- 

 ered with afferent pore-sieves (Plate 13, figs. 23, 24), which coalesce to form 

 extensive, nearly continuous poral tracts. The pores are oval or, more rarely, 

 circular, and generally measure 25-300 /i in diameter. The strands of dermal 

 tissue separating them are as broad or broader than the pores themselves. 

 These pores lead into elongate subdermal cavities, from five to seven of which 

 join to form stellate groups 0.4-1 mm. in diameter. These groups of radiating 

 subdermal cavities are sunk in the sterraster- armour layer and the spaces be- 

 tween them in great part occupied by sterrasters. In consequence of this the 

 stellate cavity groups are very conspicuous in superficial paratangential sections 

 of appropriate thickness (Plate 13, fig. 23). The cavities of each group converge 

 to a common centre and here they join to form a radial cortical canal which 

 penetrates the cortex and leads down into the interior. These radial cortical 

 canals are 1-1.4 mm. apart, and in the sections examined arc strongly con- 

 tracted, usually quite closed. They are surrounded by mantles of circular 

 fibres. These mantles increase in thickness proximally and form chones (Plate 

 13, fig. 25b), which protrude into the subcortical cavities. Around these chones 

 I have often noticed extensive dome-shaped excavations of the proximal (inner) 

 surface of the sterraster-armour. When such excavations are present the 

 chones hang down as it were from the apices of the domes and are thus situated 

 some distance above the general lower limit of the sterraster-armour layer. 

 Below this layer, in the inner zone of the cortex, a system of subcortical cavities 

 (Plate 13, fig. 25c, Plate 14, fig. 22b) extends. From these cavities the afferent 

 choanosomal canals take their rise. These canals, and also the choanosomal 

 efferents, are narrow, the choanosome appearing very solid in consequence. 

 The flagellate chambers (Plate 14, fig. 21a) are spherical or oval and small. 

 The efferent choanosomal canals lead up to and open out into extensive systems 

 of subcortical cavities which underlie the parts of the cortex bearing the uniporal 

 efferent apertures. From these cavities radial cortical canals with chones, 

 similar to the afferent ones described above, arise. On the walls of some of the 

 efferent cortical canals of the lobose specimen of var. megana I observed a few 

 large, broad, and blunt conic spines (Plate 15, fig. 7a) which protrude into the 

 canal-lumen. As these structures are rare and as I failed to find them in thin 

 sections, where they could have been studied with higher powers, I was unable 

 to ascertain their nature. Each efferent cortical canal leads up to a single 



