322 A MONOGRAPH OF THE AUSTRALIAN SPONGES, 



sometimes, particularly in the skin, such cells with three processes, 

 show a mode of the formation of muscular tissue, very different 

 from that known in the other Ccelenterata in detail, but still 

 essentially the same, (von Lendenfeld neue Aplysinidse, Kleinen- 

 bei'g Hydra.) 



A tube consisting of a dense mass of such Mesodermal cells 

 covers in some Sponges the fibres of the skeleton. These cannot 

 be considered as muscular, but rather as a sort of tissue like the 

 periost. 



Another interesting and much more differentiated form of 

 Mesodermal cells are large gland-cells, which invest the growing 

 horn-fibres of Ceraospongiae and secrete the horn substance. (F. E. 

 Schulze, Spongidse). Similar cells are met with along the outer 

 surface of the sponge. These produce a slime which protects the 

 Sponges from noxious influences from without. (Merejkovsky, 

 Eponges de la mer blanche, von Lendenfeld neue Aplysinidae.) 



These cells have the shape of round balls of Protoplasm, 

 investing the spherical nucleus from which threads of Protoplasm 

 3-10 in number take their origin, and remaining parallel with each 

 other connect the body of the cell with the surface where the 

 secretion is to be poured forth. These threads are as long or 

 longer than the main body of the cell and stand vertical on the 

 surface of secretion. 



Cells in the core of the hollow-fibred Sponges, which act like the 

 Osteaklasts, and produce the pith in the fibres of the Sponges, 

 are also derived from the Mesoderm. (von Lendenfeld das 

 Hornfaserwachsthum der Aplysiuidse.) (1) 



It appears from this that the skeleton of the Ceraspongiai is a 

 Mesodermal structure. 



The cells in the Protoplasm in which the calcareous and 

 silieious spicules take their origin, also belong to the Mesodermal 

 layer, and are of an indifferent shape. 



We now Lave to review a series of Mesodermal elements, which 

 are alike in shape at certain stages of their development, but which 

 perform very different functions. 

 (1.) Possibly symbiotic Algai, compare Brandt Chlorophyll bei Thieren. 



