118 



GEODIA AGASSIZII. 



4228, and 4551. The small dermal amphioxes form radial, tuft-like groups, 

 which arise from the sterraster-armour, traverse the dermal layer and expand 

 above (Plate 27, fig. 2), their distal ends protruding more or less beyond the 

 surface. In some specimens, as in the one from Station 3168, a section of 

 which is represented in Plate 27, fig. 2, this protrusion is very slight; in 

 others, as in a specimen from Station 4193 and in the young specimen from 

 Station 4228, the small amphioxes protrude as much as 200-280 // beyond the 

 surface. It seems that these differences in the degree of protrusion of the small 

 dermal amphioxes are, partly at least, due to differences in the state of 

 preservation and degree of shrinkage of the tender dermal layer; in the 

 well-preserved and not much shrunken specimens their protrusion is slight, 

 in specimens not so well preserved, it is great. Some small amphioxes, 

 similar to those forming the tufts in the dermal layer, are occasionally 

 observed in the proximal layer of the cortex and in the distal part of the 

 choanosome. The cladomes of the orthoplagiotriaenes generally lie at the 

 limit between the cortex and the choanosome; their clades extend para- 

 tangentially in this level, their rhabdomes are directed radially inwards. 

 Sometimes, particularly in the young specimen from Station 4228, the ortho- 

 plagiotriaene-cladomcs are situated a little higher, within the sterraster-armour 

 layer (Plate 32, figs. 8, 11, 12). The orthoplagiotriaenes do not protrude 

 beyond the surface and take no part in the formation of the fur. The rare 

 mesoclade and amphiclade orthoplagiotriaene-derivates and the quite irregular 

 forms belonging to this category of spicules have been found only in spicule- 

 preparations, so that I am unable to say what position they occupy in the sponge. 

 I have found mesorthotriaenes in the spicule-preparations of the specimens 

 from Stations 2978 and 4199, amphiclade orthoplagiotriaene-derivates in such 

 preparations of the specimens from Stations 3168 and 4199 and the young speci- 

 men from Station 4228. The mesoprotriaenes, the anatriaenes, and their 

 various derivates are also radially arranged. The cladomes of a few of them lie 

 a short distance beneath the surface, most of them protrude freely beyond it. 

 These spicules form the principal part of the spicule-fur. In this fur the meso- 

 protriaenes are generally much more numerous than the anatriaenes. Ana- 

 diaenes and other, irregular anatriaene-derivates have been observed only in the 

 specimens from Stations 3168 and 4228. 



The small strongylosphaerasters form a single but dense layer on the surface 

 of the dermal membrane (Plate 26, fig. 15) and also occur in the interior. The 

 large oxysphaerasters, which appear to be much more numerous in the specimens 



