198 



GEODIA MEDIA. 



situ in the sections, have been observed in the spicule-p reparations. One of 

 these, an anatriaene, is, in all probability, proper to the sponge. 



Among the large choanosomal amphioxes, two forms, a slender and a stout 

 one, can be distinguished. These are, it is true, connected by transitions, but as 

 the latter are not at all numerous and as the slender amphioxes are longer than 

 most of the stout ones, and therefore cannot be considered as young forms of the 

 latter, I am inclined to consider the slender and the stout amphioxes as distinct 

 forms. 



The slender, choanosomal amphioxes (Plate 16, figs. 3-5a) are quite frequent 

 in the digitate form but rare in the massive form. They are slightly curved, 

 fairly isoactine, not very sharp pointed, 1.3-1.5 mm. long, and 23-30 fj. thick. 



The stout choanosomal amphioxes (Plate 16, figs. l-5b, 6z, 8b, z, llz) are 

 very numerous in both forms. They are slightly curved, fairly isoactine, and 

 gradually attenuated to the not very sharp-i^ointed ends. In the digitate form 

 they are 1.2-1.55 mm. long and 33-51 fx thick, in the massive form 1.3-1.7 mm. 

 long and 35-51 /x thick. 



In Bowerbank's type, reexamined by me, I found choanosomal amphioxes 

 1-1.5 mm. by 18-50 fx. Sollas gives their dimensions as 1.51 mm. by 32 p.. 



The large choanosomal styles (Plate 16, figs, llf, 13f) are not numerous. 

 They are relatively more frequent in the massive than in the digitate form. 

 These styles are 0.9-1.3 mm. long and 30-50 /x thick. Some are for the greater 

 part of their length cylindrical, and simply rounded off at one end, others some- 

 what attenuated towards, others again slightly thickened at, the rounded end. 

 In four styles of the massive form the thickness was: — 



In the centre 36 /<. at the rounded end 24 /i 



" " " 40 " " " " " 43 " 



" " " 43 " " " " " 50 " 



" " " 50 " " " " " 40 " 



These spicules are not mentioned by Sollas; I found a good many, however, 

 in Bowerbank's type. 



Both the digitate and the massive forms are exceedingly rich in irregular, 

 angularly bent or branched derivates of the amphioxes and styles described above. 

 In their dimensions these spicules agree with the regular forms from which they 

 are derived. The simplest form of amphiox-derivate is an amphiox very slightly 

 angularly bent near one end. This bend may be in the same direction as the 

 curvature of the spicule (Plate 16, fig. ly), or in a direction opposite to it (Plate 

 16, fig. 9y). In other amphiox-derivates of this kind the angular bend is much 

 more pronounced, the angle between its two limbs being smaller. Such a 

 spicule is represented on Plate 16, fig. 9c. 



