152 



GEODIA MESOTRIAENELLA. 



Intact mesoprotriaene-claclomes are very numerous in the proximal portion of 

 the fur, a short distance above the surface. The spicules forming the distal 

 (superficial) portion of the fur are mostly broken off. So far as I could see, most 

 of these terminally broken spicules are anatriaene-rhabdomes. In the dermal 

 layer small, more or less radially arranged rhabds, mostly styles attenuated 

 towards both ends, occur. Their proximally situated rounded ends are im- 

 planted in the sterraster-armour ; their pointed distal ends protrude a short 

 distance beyond the surface. Small strongydosphaerasters form a thin but 

 rather dense and continuous layer on the outer surface (Plate 34, figs. 20, 25) 

 and also occur in the lower parts of the dermal layer. Large oxysphaerasters 

 are imbedded in the walls of the cortical and subcortical canals. Oxyasters are 

 scattered in large numbers throughout the choanosome. Ever^^here, except 

 in the vicinity of the radial canals, which traverse it, the middle layer of the 

 cortex is occupied by dense masses of sterrasters. Many sterrasters, particu- 

 larly young forms, also occur scattered in the choanosome. It is to be noted 

 that the sterrasters are not nearly so numerous in the lower layer of the cortex 

 and the distal part of the choanosome as in the proximal part of the latter. 



The large amphioxes (Plate 35, fig. 31c) are slightly curved, gradually atten- 

 uated towards the pointed ends, and isoactine or anisoactine. Anisoactine forms 

 were chiefly found among the stouter amphioxes. The amphioxes are 2-2.6 

 mm. long and 20-50 /j. thick. 



The minute dermal rhahds are 196-260 fx long and 4-5 fi thick. The greater 

 number of them are styles with attenuated rounded ends. In some of these 

 rhabds this attenuation is so great that the proximal "rounded" end is hardly 

 less pointed than the distal. These spicules appear as anisoactine amphioxes. 



The orthotriaenes (Plate 35, fig. 31a) have a fairly straight, conical rhabd- 

 ome, usually sharp pointed, very rarely rounded at the acladomal end. The 

 rhabdome is 2.1-2.4 mm. long and, at its thickest point, 75-120 p. thick. This 

 thickest point is situated either at the cladome or a short distance below it, and, 

 in the latter case, separated from the cladome by a slight neck-shaped con- 

 striction. The clades are always conical and curved, concave to the rhabdome 

 in their entire length; the degree of curvature and the size of the clades are, 

 however, variable, the clades of the same cladome often differing from each other 

 considerably in these respects. The clades are sharp pointed or, more rarely 

 rounded at the ends, and generally simple, but cladomes with one or two bifur- 

 cate clades have also been observed occasionally. The clades are 350-600 ft 

 long and their chords enclose angles of 90-96°, on an average 93°, with the axis 

 of the rhabdome. 



