GEODIA ]\IESOTRIAENA. 99 



Plate 23, figs. 24c, 25c; Plate 25, fig. Ic) 120-300 /( wide. .411 the canals 

 originating from the pores of the same group converge to a point below 

 its centre and there join to form a cavity of considerable extent, which lies in 

 the outer layer of the cortex below the dermal membrane (Plate 23, figs. 24, 25). 

 From this subdermal cavity a narrow tube, the chonal canal (Plate 22, figs. 3-7c; 

 Plate 23, figs. 24, 25k) extends radially downwards, penetrating the middle and 

 inner layers of the cortex. This chonal canal, particularly its proximal part, 

 which passes through the inner layer of the cortex, is surrounded by a ring of 

 contractile tissue, the chone (Plate 22, fig. 6; Plate 23, figs. 24d, 25d), accord- 

 ing to the degree of the contraction of which the width of the chonal canal 

 varies. In most of the sections it is about 200 /( wide above, where it enters 

 the middle cortical layer, and narrows centripetally to about 50 /( at its proximal 

 end (Plate 22, figs. 3-6; Plate 23, fig. 25). 



At the lower limit of the inner layer of the cortex, the chonal canal opens out 

 into a wide choanosomal canal, likewise radial,' extending down towards the in- 

 terior. These radial choanosomal canals (Plate 23, figs. 24, 25e) are usuall}^ 

 300-600 n, sometimes as much as 1 mm. wide. Transverse membranes, protrad- 

 ing from their walls at intervals of 150-250 fi, partially divide the canal-lumen 

 into a row of chambers. Some of these ratlial canals are short and soon split 

 up into numerous narrow branch-canals which lead to the superficial flagellate 

 chambers, others are long and open into larger canals, 3-5 mm. wide, which 

 extend in a paratangential or oblique direction into the deeper parts of the 

 choanosome (Plate 21, fig. 2). Some of the radial canals leading down from 

 the chones are surrounded by thick mantles free from flagellate chambers (Plate 

 23, figs. 24f, 25f), which appear as centripetal continuations of the tissue form- 

 ing the lower cortical layer. The chamber-bearing tissue (Plate 23, fig. 24g) 

 occupies the interstices between the canals (canal-mantles). Within this tissue 

 the flagellate chambers are numerous and close together. They appear to be 

 more or less spherical and have a diameter of 20-30 /.«. 



It will be seen by the above that in general structure and in the character 

 of its canal-system this sponge is very similar to the Mediterranean Geodia niulleri 

 {cydoniuni)} I have been able to examine this species in various stages of 

 growth and thus to ascertain the true nature of the tubes leading down from the 

 large depressions on the surface and the different parts of the canal-system proper. 

 I think there can be little doubt that in G. mesotriaena, as in G. mtlUeri, both the 



' R. V. Lenrleiifelil. Die Tetractinelliden der Atlria. Denk. Akad. wissensch. Wien, 1S94, 61, p. 

 1.38, ff. 



