172 



GEODIA MICROPORA. 



the pore-sieves are surrounded by a ring of five or six regularly distributed 

 spicule-tuf ts ; the intervals between these tufts are equal. 



Hardly any euasters are observed in the interior of the choanosome; distally 

 large oxyasters are met with in small numbers. In the subcortical layer and in 

 the walls of the radial cortical canals large oxysphaerasters occur. The dermal 

 membrane is occupied by small strongylosphaerasters. These are more numer- 

 ous between the pore-sieves than in the narrow bands separating the individual 

 pores from each other. A large number of sterrasters, chiefly young forms, 

 occur in the interior of the choanosome. In its distal part there are only few 

 sterrasters. In the proximal part of the sterraster-armour layer the sterrasters 

 are quite far apart; in its distal part they are closely packed together and form 

 a dense mass (Plate 37, fig. 3). 



Some of the large amphioxes are simple, others centrotyle. Forms with 

 a hardly perceptible central thickening connect the centrotyle forms with the 

 simple ones. First I thought that the centrotyle amphioxes were young stages 

 of the others; since, however, centrotyles are found among the thickest of these 

 amphioxes and simple, non centrotyle ones among the thinnest, I hardly think 

 that this can be so. The large amphioxes are 1.2-1.6 mm. long, in the middle 

 20-28 fi thick, and usually slightly curved. The tyle is always situated in the 

 thickest, central part of the spicule. The diameter of the properly developed 

 central tyle is, as the following measurements show, 12 to 30 % greater than the 

 thickness of the adjacent parts of the spicule. The thickest tyle observed 

 measured 31 // in transverse diameter. 



Thickness of amphiox close to the 

 central tyle. 



21 n 



22 " 



22 " 



23 " 



23 " 



24 " 



Transverse diameter of the 

 central tyle. 

 27 fi 



25 " 



26 " 



26 " 

 31 " 



27 " 



The minute dermal rhabds (Plate 36, figs. 24a, 26a, 27a) are amphistrongyles 

 attenuated towards both ends. They are generally isoactine, rarely anisoactine, 

 usually slightly curved, and 125-165 p. long. In the middle they are 2-3.6 pt, 

 at the ends 1.8-2 fi thick, the thickness of the ends being on an average about 

 59 % of the maximum thickness in the middle. 



The orthoplagiotriaenes (Plate 37, figs. 4-7) have a fairly straight, conic 

 rhabdome 1.1-1.45 mm. long and, at the cladome, 28-47 fx thick. The acladomal 

 end is usually blunt pointed. Sometimes irregular rhabdomes (Plate 37, fig. 



