560 STUDIES ON SPONGES, 



average 0'4 mm. apart. When fully dilated they reach a width of 

 0*2 mm. After staining every pore appears surrounded by a dark 

 ring. (Fig. 2.) More highly magnified (fig. 3) this ring dissolves 

 itself into a number of irregular roundish and multipular cells 

 which are situated a little distance away from the free margin. 

 Between the margin and these cells granular threads, vertical to 

 the contour of the margin are found, some of these contain a highly 

 stained slightly elongate nucleus. The nuclei of the cells which 

 form the ring, always are spherical. 



Behind the cells of the ring, spindle-shaped muscular fibres are 

 found in abundance. They extend tangentally and often a number 

 of them combine to form a regular bundle of parallel fibres. 



Tran verse sections (figs. 4, 5) enable us to attain a clearer 

 insight into these structures. The membrane has a thickness of 

 O'l mm. The pores (fig. 4) are not cylindrical but conic holes. 

 The diameter of the pore decreases towards the interior, and it is 

 the lower side where the pores are closed when the muscular fibres 

 in the membrane contract. 



The whole of the membrane is covered by very low and flat 

 epithelium cells. (Fig. 5.) These are doubtlessly ectodermal. 

 The interior of the membrane is occupied by a gelatinous ground 

 substance, in which cells of various kind are imbedded. 



Just below the surface gland cells are met with. These are 

 present in great numbers on both sides of the lamella. They are 

 similar to the homologous elements in other si^ecies of Aplysillidse 

 described by me (1), contain a large spherical nucleus in their 

 bulbous pi'oximal portion, which is connected with the surface by 

 a number of slender processes (tig. 5). These are simple or slightly 

 ramified and always vertical to the surface. On the internal margin 

 of the pore such gland cells are not met with. Here a diSerent kind 

 of cell is found below the ectodermal epithelium. These cells are 

 slender and spindle-shaped and form a ring round the pore. In 



(1) i?. fon Lendenfefd. Ueber Ccelenteraten der Siidsee II., Mittheilung. 

 Neue AplysiuidaB. Zeitschrift fiir wissenschaftliche Zoologie. Band 

 XXXVIII. 



