BY R. VON LENDENFELD, PH.D. 321 



If we compare the Histology of the Sponges, Pol yponiedusse, 

 Actiniaria, and Ctenophora?, we shall find that the four groups 

 differ from each other in the following manner : — 



The principal oi'gans in the Ectodermal Epithel, Polypomedusse. 



The principal organs in the Entodermal Epithel, Actiniaria. 



The principal — or many. — organs in the primary Mesoderm or 

 ground substance, Sponges and Ctenophorae. 



As mentioned above, the ground substance, which is a product 

 or secretion of the Meso-dermal cells, mostly has the appearance 

 of structureless gallerts, or harder substance, which in every case 

 contains a large proportion of water. The minute fibrelles found 

 in it run in every direction and are often dense and wavy so as to 

 resemblance for instance the connective tissue in the cervical 

 teidon of a bullock. 



Cells of the Mesoderm. 



The numerous cells of the Mesoderm have very different 

 shapes. They possess a spherical or oval nucleus around which 

 the Protoplasm is amassed. These cells are destitute of a cell- 

 wall. From the central mass of Protoplasme prolongations extend 

 exceeding in length the diameter of the central part manifold. 

 These processes are not numerous 2-6. The most indifferent forms 

 have processes which may extend in every direction. With 

 a higher development of these elements their processes get fewer in 

 number, and extending along one plane only. In the case of only 

 two such excrescences being developed, they always lie at opposite 

 ends of the cell and in one line — here we have arrived at a form 

 of Mesodermal cell, which is to be termed a musular cell. 

 Near the surface of the Sponge in the membrane which divides 

 the Sub-dermal cavities from the outer water these cells have 

 their prolongations extended in a tangental plain, they are 

 mostly two in number but sometimes also three are met with. 

 Around the canals and the Oscular tube we often find these 

 cells in dense masses, and their prolongations mostly running 

 parallel to the adjacent surface. The extensive movements of the 

 outer skiu and the canals is doubtlessly the result of the contraction 

 of the processes of these cells. The interesting fact of their being 



