PLATE XXXVII. 
Siphonidium capitatum, . 
Figure 1. The sponge ; nat. size. 
,, 2-21. Spicules — 
Fig. 2. Ectosomal desma; x 292. 
„ 3-8. Varioas forms of choanosomal 
desma; x 95. Fig. 3 repre- 
sents a form frequently as- 
sumed when the desma occurs 
immediately below the ex- 
ternal surface. 
„ 9-15. A series of young desmas, ar- 
ranged in the order of age; 
fig. 9 represents an example 
scarcely advanced beyond the 
stage of crepis, in fig. 14 the 
crepis is to be seen in the 
middle of the spicule. Fig. 
9 X 200, figs. 10-12 x360, 
figs. 13, 14 X 200, fig. 15 
X 95. 
Figs. 16, 17. Three examples of the oxy- 
tylote, showing the much 
enlarged axial canal; x 200. 
„ 18-20. The tylote terminations of 
other examples ; x 200. 
„ 21. Desmas in zygosis; on the 
right lower corner of the 
figure a small secondary 
desma is to be seen; x 95. 
22. The skeleton of the terminal part of one half of one of the tubular 
processes of the sponge, cut longitudinally ; x 95. 
23. Transverse section through the wall of one of the tubes; x 73. The 
thin line tinted brown and blue situated to the right, separated from 
the rest of the section, represents the displaced outer membrane 
which bounds the exterior of the sponge. 
24. Part of a transverse section, to show the collenchymatous layer in the 
interior of the tube ; x 49. 
25. Section through the choanosome, showing flagellated chambers; x 292. 
26. Vacuolar structure seen in the collenchyma of flg. 24 ; x 292. 
27. Vacuolated tissue lying below the external membrane of the sponge ; 
X 360. 
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