THE PORIFERA. 105 



distinctness of the cells which contain these nuclei, and is 

 therefore termed by Haeckel a syncytium. It is elastic and 

 contractile, and sometimes exhibits an approach to fibrillation. 



The endoderm, on the contrary, is composed of a layer 

 of very distinct cells, each of which contains a nucleus and 

 one or more contractile vacuoles, and is produced at its free 

 extremity into a long solitary cilium or flagellum. Around 

 the base of this, the transparent outer portion of the proto- 

 plasm of the cell is produced into an upstanding ridge like a 

 collar, so that each cell has a wonderful resemblance to some 

 forms of flagellate Infusoria. Microscopic apertures — the 

 pores — scattered over the outer surface of the cup, lead into 

 short passages which perforate the ectoderm and endoderm, 

 and thus place the ventriculus in communication with the ex- 

 terior. The working of the flagella of the endodermic cells 

 causes the water contained in the gastric cavity to flow out 

 of the osculum ; to make good this outflow, minute streams 

 set in by the pores, which have consequently been called in- 

 halent^ while the osculum has been termed the exhalent aper- 

 ture. It is said, however, that the direction of these currents 

 is not invariable ; and it is certain that the pores are not 

 constant, but that they may be temporarily or permanently 

 closed, and new ones formed in other positions. 



The skeleton of the calcareous sponges always consists of 

 a multitude of separate spicula, composed of an animal sub- 

 stance, more or less strongly impregnated with carbonate of 

 lime, which is deposited in concentric layers around a central 

 axis, formed by the animal basis. This skeleton is devel- 

 oped exclusively in the ectoderm, and is not supported by 

 any framework of fibrous animal matter. 



The calcareous sponges are frequently, if not always, 

 hermaphrodite. The reproductive elements are ova and 

 spermatozoa. There is some reason for assuming that the 

 latter originate in metamorphosed cells of the endoderm, as 

 they are found scattered between ordinary cells of the latter. 

 The ova, on the other hand, occur sometimes between the 

 cells of the endoderm, sometimes imbedded in the syncytium 

 itself. But the question of the origin of the sexual elements 

 in these and other animals needs much further investigation. 

 The spermatozoa are very delicate, and have minute, rod-like 

 heads, with long flagella. The ova present the normal ger- 

 minal vesicle and spot, but exhibit active amoeboid move- 

 ments. 



Impregnation is eflfected, and the first stages of develop- 



