4 Porifera. 



wover-developed velum. Beneath the cortex there are the well-known crypts, 

 which Marshall believed to be homologous with subdermal cavities. Vosmaer has 

 thrown doubts on this equivalence, but S. seems to be somewhat of Marshall's 

 view. In two cases the evidence seems fairly clear. In young examples of S. 

 phrissens the choanosome is folded within a thin ectosome. The outer sinuses of 

 the choanosomal folds represent both subdermal cavities and incurrent canals. <t 

 Later the ectosome is converted into a comparatively thick cortex, which is in- 

 terrupted by vertical canals (the future chones) . The subcortical crypts arrise 

 from the incurrent sinuses, and are clearly homologous with the subdermal ca- 

 vities of Tetilla and other sponges. In Craniella simillima the cortex of the 

 youngest observed specimentc is subdivided into two regions, one of which is 

 continuous with the choanosome, from which it evidently arises by metamorpho- 

 sis . The other region forms a thin investing membrane, separated from the 

 former region by wide cavities, which are evidently the subdermal cavities. 

 The arrangement of the main canals S. calls, like Zittel did, the type of canal- 

 system; what Vosmaer called so, he replaces by type of chamber-systems. The 

 simplest type of canal-system occurs in Plakina monolopha F. E. S. and Oscaretta 

 lobularis. The nearest approach to it is made by Epallax callocyathus. As in 

 most other plate-like sponges, the oscules are to be found on one side, the pores 

 on the other. A transverse section reveals two, apparently ectosomal, layers and 

 between them a regularly folded sponge-plate. But the folds are folded them- 

 selves, the flagellated chambers opening into the sinuses of the secondary folds. 

 In how far this sponge can be derived from aRhagon, is doubtful; embryological 

 evidence is absent. If the epithelium of the oscular face is endodermal, the ca- 

 vity of the vase-like sponge represents the paragaster of the Rhagon, and is 

 called endosome. If, on the contrary, the oscular face is ectodermal, its super- 

 ficial layer consists of an outer ectodermal, an inner endodermal epithelium and 

 a mesodermal layer between. It then represents ahypophare. In young or small 

 spherical sponges, the axes of evagination radiate towards the periphery, but in 

 full-grown examples this is hardly to be seen. There seems to be however a 

 tendency to a radiate and a concentric arrangement of the canals. Something 

 similar we find in Siphonia and Petrosia. The type of canal-systems of Thenea 

 is a modification of the preceding. The symmetrically arranged canals of Disy- 

 ringa dissimilis and the closety allied Agilardiella originate by a regular folding of 

 the choanosome. Here the symmetry is of secondary origin and by no means of 

 importance, when considering the relation to Coelenterates. - -Histology. The 

 ectoderm occurs as an epithelium of pavement- cells (pinacocytes) . It invests 

 the exterior of the sponge and lines the intercortical cavities and incurrent canals. 

 The pinacocyte of the endoderm differs in no perceptible respect from that of the 

 ectoderm. A cuticula does not exist. The distal ends of the collars of the choa- 

 nocytes become concrescent with those of the surrounding choanocytes. From 

 this results a delicate film, which forms a second lining to the wall of the flagel- 

 lated chamber [see Bidder, supra, p 2]. In Suberitidae and Tethyidae this 

 arrangement is not found. The niesoderm in its simplest form consists of 

 gelatinous connective tissue (collenchyma; ; i. e. a transparent jelly-like basis, 

 produced by the alteration or secretion of stellate cells (>icollencytes) . Modifi- 

 cations of collenchyma are : sarcenchyma (with a beginning of granulation) , 

 cystenchyma'< (vesicular) , chondrenchyma (resembling hyaline cartilage) . For 

 various cells of the connective tissue S. has invented new names. Thus the- 

 socytesc are such modified collencytes, which were known as reserve cells ; 

 chromatocytes = pigment-cells, inocytes = fusiform connective-tissue cells, 

 myocytes = muscle-cells, oaesthocytesc = nerve-cells. Especial attention 



