340 THE AMERICAN MONTHLY [Pec, 



lowed after a ratlier sliort time by the closure of the sphincter. 



Supposed stMise cells, o^sthacvtes. hnve been observed by 

 Stewart aiul have been described l>y Lendenleld, they are s{)in- 

 dle shaped, one end projects through the epithelial ectoderm, 

 the other eml becomes continuous with cells that TiCndenfeld 

 supposed to be multij)olar uanglion cells. Other cells the same 

 observer regards as nerve cells and muscular cells, but these 

 liave not as yet been sufficiently studied. Pigment cells are 

 generally distributed throughout the sponges, but more especi- 

 ally below the ectoderm. 



Each spicule of a sponge originates in a single cell, tlic 

 scleroblast. When the spicule has comj)leted its full growth the 

 cell atrojihies. During its growth the spicule slowly i)asses 

 from the interior to the exterior of the sponge, and is finally 

 (in at least some sponges, Geodia, Slellala), cast out as a effete 

 product. The horny fibres of the Keratoma are j)roduced as 

 the secretion of cells known as sponginblasts. which as a 

 continuous mantle surrounds each growing fibre, and cover each 

 growing point in a thick cap. The lateral sponginblnsts are 

 elongated radially to the fibre; the terminal sponginblasts are 

 polygonal and depressed, the latter giving rise to the soft gran- 

 ular core, and the former to the spongin wall of the fibre 



New individuals are pro<luced a.sexually by both internal and 

 external gemmation, by fission, and by sexual reproduction. 

 Fission is probably one of the processes by which compound 

 sponges are i)roduced from simple individuals. External g-^m- 

 mation has been obs rved in diflferents species. A mass of in- 

 (fifferent sponge cells accumulate at some point beneath the 

 skin, bulges out, drops off, and gives rise to a new individual. 

 Internal gemmation, which results in the formation of stato- 

 blasts (or gemmules) is only known to occur in fresh-water 

 sponges (Spongillidaj). These statoblasts are formed at the ap 

 preach of winter in cold climates, or at the approach of drought 

 in warm climates. They consist of a mass of yolk bearing mes- 

 oderm cells, invested by a double capsule, at the side of which 

 is a small aperture, the hilum. The walls of the capsule are 

 supported by small siliceous spicules called Amphidisks. In 

 spring tmie, or at the return of the wet season, the interior cells 

 creep out through the hilum, and by differentiation give rise to 

 a young spongilla. 



