38 JOURNAL OF THE 



the Mediterranean, on the under side of stones, shells, etc. 

 In the simplest species, P. inonolopJia^ there is a continuous 

 basal cavity crossed by strands of tissue. From the cavity 

 run more or less vertical efferent canals, which are simple 

 or very slightly branched, and into which open the flagel- 

 lated chambers. The afferent canals are spacious cavities 

 opening on the surface by wide mouths. The periphery 

 of the sponge forms a continuous rounded rim, the "ring- 

 wall," and the oscula, one or several, are situated here. 

 The surface of the sponge inside the "ringwall" is divided 

 up into low rounded elevations, caused by the upper ends 

 of the efferent canals, between which lie the wide apertures 

 leading into the afferent canals. Schulze was fortunately 

 able to observe the main features in the development of 

 this interesting form. There is a solid swimming larva 

 which settles down, forming a flat circular mass. A cen- 

 tral cavity appears in the mass, the lining cells becoming 

 columnar, and the sponge is thus transformed into a flat, 

 three- layered sac, the three layers being respectively ecto- 

 derm, mesoderm, entoderm. The flagellated chambers 

 appear in a single layer round the central cavity into which 

 they open. They are very probably formed as diverticula 

 of this cavity. Schulze did not follow the development 

 further, but a comparison of the adult with the sac-like 

 young form makes it pretty certain that the young form 

 undergoes a process of folding, which gives rise to the 

 efferent and afferent canals of the adult; or, in other words, 

 the efferent canals arise as vertical e vagi nations of the sac- 

 like stage. The afferent canals are consequently to be re- 

 garded as lined with ectoderm. 



In the other two species [P. dilopha and P. trilophd) the 

 oscula are not situated at the periphery as in P. Dioiiolopha^ 

 but at some distance internal to it; and the efferent canals 

 do not form projections on the surface as in the first species. 

 A comparison of the canal systems makes it evident that 



