170 



GEODIA MICROPORA. 



in G. miilleri, their place here being taken by strongylasters much smaller in 

 size. 



Apart from the minute anatriaenes the megascleres of G. mesotriaena are 

 in shape and size very similar to those of G. ovis. The former is, however, dis- 

 tinguished from the latter by the possession of praeoscular cavities, the lower 

 spicule-fur, and the absence of minute anatriaenes. Its sterrasters are larger, 

 its dermal asters much smaller, and the large oxyasters with stout, regularly 

 conical, and sharp-pointed rays, so abundant in G. ovis, are wanting in G. meso- 

 triaena. 



I establish this species for a specimen from Duncan Island, Galapagos. 



The radial cortical canals, both the afferents and the efferents are very 

 narrow and their entrances, although covered with pore-sieves, appear to the 

 naked eye as very small, simple, open pores. To this the specific name refers. 



Shape and size. The specimen (Plate 36, fig. 32) appears as a part of a 

 larger lobose mass, measures 63 mm. in maximum diameter, and is attached 

 to some black pebbles and pieces of coral. The surface is fairly smooth. On 

 its protected parts remnants of a spicule-fur are observed. With the excep- 

 tion of a belt about 1.5 mm. broad, which extends for a considerable distance 

 and in which there are only few minute pores, the whole of the surface is 

 occupied by the small cribriporal entrances to the radial cortical canals. On 

 one side of the belt the entrances to the cortical canals are 50 -200 /x wide and 

 their centres on an average 350 n apart; on the other side they are 200-500 // 

 wide and their centres on an average 600 /j. apart. The small canal-entrances 

 on the one side of the belt I consider as afferents, the wider ones on the 

 other side as efferents. 



The colour, in spirit, is brownish white, somewhat lighter on the surface 

 than in the interior. 



The superficial part of the body forms a cortex (Plate 37, figs, la, 2a, 3) 

 450-600 /X thick. This is composed of three layers, an outer dermal layer (Plate 

 37, fig. 3a) 60-101 fi thick, a middle sterraster-armour layer (Plate 37, fig. 3c) 

 300-420 f.1 thick, and an inner fibrous layer 45-100 thick. The latter is 

 occupied by paratangential fibres and by cells strongly staining with haema- 

 toxylin. Most of these cells are elongated and extend paratangentially. 



Canal-system. The afferent area of the surface is covered with pore-sieves 



Plate 36, figs. 13-36; Plate 37, figs. 1-14. 



