PLACOSOMA tARADICTYUM. ^ 



Oil its surface. Length of transverse axis, 243 mm. Breadth in 

 vertical direction, 126 mm. Thickness in the middle, 92 ram. 



On account of the compression tlie Ixxly presents two sides, 

 both with uneven, thougli on the whole convex, surfaces. The 

 margin is bv no means thin, being in places quite thick and 

 rounded, while in other places it shows an angular edge-line. 



Very remarkable and striking is the difference in appearance 

 presented by the two sides of the sponge- body. Nevertheless, 1 

 consider the surfaces of both sides, in fact the entire external 

 surface, as dermal, and therefore as adapted to the afferent passage 

 of water. However, as judged from structural relations, the 

 inflow evidently takes place with special activity, in fact principally, 

 on the one side which may be called the front (PI. I., fig. 1), 

 while the other side, the back (fig. 2), seems to allow it only in 

 a relativelv insic;nificant deo;ree and is further characterized bv 

 the presence of numerous oscula distributed over it. 



The front of the entire sponge is quite destitute (Ä oscula. So 

 far as the smooth and undulating frontage of the l)ody proper is 

 concerned, it is nearly, if not wholly, occupied l)y a specialized 

 area of the dermal layer, the frontal lattice, which consists of 

 open-meshed dermal and hypodermal latticeworks of exquisite 

 beauty. The dermal latticework (PI. II,, fig. 13), in which the 

 laths are supplied by the fine paratangential rays of dermalia, is 

 exceedingly delicate and shows small, regularly quadrate meshes. 

 These measure 150-240 fi in length of sides. In nature they are 

 easily discernible with the naked eye, but not in the reduced 

 fig. 1, PI. I., in which however the minutely tessellated pattern 

 may in some parts be observed with the aid of a hand-lens. — 

 The hypodermal latticework, plainly visible in the figure just 



