PORIFERA. 801 



progress of segmentation 1 . The posterior pole or the anterior (Reniera), 

 sometimes both poles, are marked by a pigment patch. The larva is very 

 sensitive to light, and as a rule attracted by it, more seldom repelled 

 (Renter a). In Chalinula fertilis mesoglaea appears among the cells of 

 the anterior and middle portions of the inclosed mass, and spicules, as is 

 commonly the case, are formed in some of the cells near the periphery. 



Fixation takes place by the gastrula mouth, which is subsequently 

 closed in Sycandra (Sycon] raphamis and Oscarella lobularis 2 ; by the pro- 

 jecting cell-mass at first in Chalinula fertilis, afterwards by one of the two 

 sides of the larva which has now become flattened ; by the posterior pole 

 in Plakina ; by the central mass protruding at the posterior pole in Reniera 

 filigrana. The ectoderm cells throw out short pseudopodial processes for the 

 purpose of fixation, which are retracted as development progresses ; they 

 lose their columnar form and are gradually flattened. The stages sub- 

 sequent to the larval have as yet been accurately observed only in a few 

 instances. Sycandra (Sycori) raphanus becomes cylindrical with a flattened 

 free pole, in the centre of which the osculum is formed. Before it appears, 

 however, spicules are developed in mesoglaeal cells, and then protrude at 

 the sides of the cylinder, which are pierced by pores. At the same time 

 the endoderm cells, which lost their collars and flagella by absorption 

 immediately after invagination, acquire them again. 6". ciliata (Sycon 

 ciliatiuri) may attain a height of I mm. before the radial cones begin to 

 form as evaginations of the wall. The endoderm cells of the central tube 

 flatten out and lose their flagella at a later period. In Oscarella lobularis 

 the gastric cavity or archenteron developes a peripheral circle of diverticula, 

 some large, some small, parallel to the surface of the sponge, and directed 

 aborally. These diverticula are in the course of growth thrown first into 

 radial, i. e. zig-zag folds ; the folds are then divided by constrictions into an 

 outer and inner circle of chambers or ampullae, and the outer circle is 

 doubled in number by the radial division of each ampulla. A cross section, 

 however, shows that the ampullae form a single superficial layer, a condition 

 which persists in the adult, and is seen also in some Plakinidae. The 

 ampullae open into the central gastric cavity, the cells of which are now 

 cubical and flagellate but no longer collared, by wide mouths subsequently 



1 The anterior pointed pole and the posterior concave pole of the conical larva are said to be 

 sometimes non-ciliate, and the latter to be girdled by cilia of exceptional length. But Schulze found 

 the concave pole ciliated in Spongelia and Euspongia, and Keller states that in Chalinula fertilis it 

 is non-ciliate before the escape of the larva, that it then acquires cilia but loses them just before 

 fixation. In Reniera filigrana, according to Marshall, the central mass forces its way out at each 

 pole of the oval larva when it is mature, and the anterior bare patch is surrounded by very long 

 cilia. 



a Oscarella is attached in later stages by one or two foot-like processes ( = tentacles ?), and 

 here and there by the base of the sponge, i. e. by the surface formed on the closure of the gastrnla 

 mouth. Heider states that the ectoderm covering the feet is columnar and secretes a clear cement. 



3F 



