188 GEODIA ACANTHTYLASTKA. 



Geodia acanthtylastra, sp. nov. 

 Plate 45, figs. 1-39; Plate 46, figs. 1-21; Plate 47, fi?s. 1-8. 



I establish this species for five spirit specimens from the coast of Lower 

 CaUfornia (Station 2829). The choanosomal asters are chiefly acanthtylasters 

 and to this the name refers. 



Apart from differences in age which find their expression in differences of 

 size, the five specimens are identical in structure. They are (Plate 46, figs. 16, 

 29) irregularly spherical, oval, or tuberous, the smaller ones being more regular 

 than the larger. The smallest specimen measures 10 by 8 mm., the largest 20 by 

 13 mm. In sheltered places, chiefly in the vicinity of the base of attachment, 

 remnants of a spicule-fur are observed. The surface is minutely pitted. The 

 pits, which mark the positions of the entrances to the radial cortical canals, 

 appear in some specimens to be uniformly distributed and everywhere a little 

 under 0.5 mm. apart. In other specimens the pits are larger and more distant, 

 in one or two restricted areas on an average 1 mm. apart. Larger apertures, 

 visible to the unaided eye, do not occur on the surface. One of the specimens 

 is partly overgrown by a thin crust of a monaxonid sponge. 



The colour, in spirit, is brownish white. 



The superficial part of the body is differentiated to form a cortex (Plate 46, 

 fig. 16). This is composed of three layers: a dermal layer (Plate 46, fig. 39a; 

 Plate 46, fig. 20b) 40-160 n thick; a middle sterraster-armour layer (Plate 46, 

 fig. 39b; Plate 46, fig. 20c) 150-350 n thick; and a fibrous inner layer. This 

 inner layer is very thin and quite inconspicuous. In many places the chamber- 

 bearing choanosome extends right up to the sterraster-armour; in these the 

 inner layer cannot be made out at all. 



Canal-system. The dermal membrane is perforated by groups of pores. 

 These pore-groups occupy the pits mentioned above. The pores themselves 

 are oval, in some places 10-40 fi, in others (Plate 46, fig. 21) 20-70 /t wide. The 

 smaller appear to be the afferent, the wider the efferent pores. 



The pores of each group lead into a system of wide, lacunose canals (Plate 

 45, fig. 39d) excavated in the dermal layer, which converge and join to form a 

 radial cortical canal. The radial cortical canals observed were all either strongly 

 contracted or quite closed. 



The flagellate chambers (Plate 46, fig. 28b) appear oval in sections and have 

 a maximum diameter of 20-25 fi. Numerous rather wide canals traverse the 

 choanosome (Plate 45, fig. 16). 



