SPONGES. 67 



widely distributed in certain tetractinellid and monaxonid groups, may have originated 

 from such microxea as we find here, in other words, from diactinal asters. 



The distinction hetween ectosome and choanosome is not well-marked, and there 

 is no properly defined cortex. The ectosome is, however, fairly thick (about 

 0'27 millim.), and much more densely spicular than the choanosome ; it contains 

 numerous small brown pigment cells (which are also met with in the outer part of the 

 choanosome). There is no fibrous tissue in the ectosome, and what little mesogloea 

 there is between the densely packed spicules is probably collenchymatous. The 

 mesogloea of the choanosome is very finely arid uniformly granular. 



The dermal pores lead into short inhalant canals which penetrate the ectosome 

 more or less vertically and, after uniting together to a greater or less extent, open 

 beneath the ectosome into spacious " crypts," from which the inhalant canals of the 

 choanosome take their origin. The flagellate chambers are rather large, pouch- 

 shaped, about 0'04 millim. in longer diameter; eurypylous, or with short, wide 

 exhalant canals. 



RN. 139, 235 (? Parts of the same specimen. From Station XLI., 12 miles off 

 Galle, 100 fathoms). 



SUB-ORDEH: ASTROPHORA. 



Tetractinellida with trifenes and with astrose microscleres ; without sigmata. 



FAMILY : PACHASTEELLID^E. 



Astrophora without long-shafted trisenes and without sterrasters. Calthrops, in 

 addition to short-shafted trisenes resembling calthrops, may be present. 



Plakinastrella, SCHULZE. 



Pachastrellidee with calthrops and (or) short-shafted trisenes and oxea for megascleres ; 

 and oxyasters and microxea for microscleres ; the microxea forming a special 

 dermal skeleton. 



This genus occupies an intermediate position between the Plakinidae and the 

 Stellettidse, and probably indicates the first stage in the evolution of the lonu'-sliafted 

 tricene from the calthrops. The relationship to the Plakinidse is clearly shown by the 

 form of the oxea, and the differentiation between megascleres and microscleres is not 

 yet by any means complete. 



Plakinastrella intermedia, n. sp. Plate I., fig. 4 ; Plate II., fig. 2. 

 Specimen (Plate I., fig. 4) irregularly branched and shortly stalked ; branches short, 

 thick, irregularly nodose, spreading more or less horizontally, rounded at the extremi- 

 ties. Surface smooth but uneven, minutely porous in parts. Texture firm and 

 incompressible, with many relatively large foreign bodies embedded in the substance 



K 2 



