1 62 



PHYLUM PORIFERA — SPONGES 



and is doubtless general. Segmentation is total and usually 

 equal, and results in a spherical or oval embryo more or 

 less flagellate. This leaves the parent sponge, swims about 

 for a time, then settles down, and undergoes a larval meta- 

 morphosis often diflicult to understand. It is peculiarly 



difficult to bring the history of the 

 germinal layers in sponges into 

 line with that in other Metazoa. 



{a) In the small calcareous sponge 

 Sycandra raphanus (Fig. 78), of the 

 Mediterranean, whose development is 

 very similar to that of the common 

 British Sycon ciliatum, the segmentation 

 results in a hollow ball of cells — the 

 blastula. A few cells at the lower pole 

 remain large, and are filled with nutritive 

 granules ; the other cells divide rapidly 

 and become small, clear, columnar, and 

 flagellate. The large granular cells be- 

 come invaginated, at first, \mtil the 

 embryo is free of the parent, when the 

 rounded form is restored in the amphi- 

 blastula (Fig. 78, 4). This swims for a 

 time actively, but the flagellate cells of 

 the upper hemisphere are invaginated 

 into or overgrown by the large granular 

 cells, and thus what is generally called 

 the gastrula stage results. This soon 

 settles down, on rock or seaweed, with 

 the blastopore or gastrula mouth down- 



FiG. 79.— Diagrammatic re- wards, and is moored by amoeboid pro- 

 presentation of develop- messes from the granular cells, which 

 ment of Oscarella lobularis. likewise obliterate the blastopore. The 

 After Heider. granular cells lose their granules, for the 



Bl., Free-swimming blastula with larva is not yet feeding ; the flagellate 

 flagella ; G., gastrula settled ^ells begin to acquire the characteristic 



NexTTgure shows folding of inner collar ; a mesoglcea with spicules begins 

 layer (En.) ; Ec, outer layer. to be formed between the inner and outer 



Lowest figure shows radial cham- layer, probably by migrants from the 



bers {R.C.) ; Mesoglcea (Mg.) ; i„4.*p_ Pores ooen through the walls 

 inhalant pore (P.) ; exhalant ^atier. rores open xnrougn ine waus, 



osculum (0.). water is drawn m by the action of the 



flagella, and an exhalant aperture is 



ruptured at the upper pole. The young sponge i^ now in an A scon 



stage, from which, by the outgrowth of the inner flagellate layer into 



radial chambers, it passes into the permanent Sycon form, heightens, 



and becomes differentiated in detail. 



(b) In Oscarella (Halisarca) lobularis (Fig. 79), a sponge without any 



skeleton belonging to the Demospongise, the ovum seements equally 



into a blastula, which is flagellate all over. During this free-swimming 



