ISO PORIFERA 



by returning into the tissues of the mother sponge, before the 

 columnar cells have completed the chitinous coat. Yet another 

 migration now occurs, the cells — " scleroblasts " — which have been 

 occupied in secreting amphidiscs at various stations in the sponge, 

 carry the fully formed spicules to the gemmules and place them 

 radially round the yolk cells between the radially lying cells of 

 the columnar layer. The scleroblasts themselves remain with the 

 amphidiscs, and becoming modified, contribute to the formation of 

 the air-chamber layer. The columnar cells now creep out between 

 the amphidiscs till their inner ends rest on the outer ends of 

 these spicules. They then secrete the outer chitinous coat and 

 return to the mother sponge.. 



Carter gives directions ^ for obtaining young sponges from 

 the gemmules. The latter should be removed from the parent, 

 cleaned by rolling in a handkerchief, and then placed in water 

 in a watch-glass, protected with a glass cover and exposed to 

 sunlight. In a few days the contents of the gemmule issue from 

 the foramen and can be seen as a white speck. A few hours 

 later the young sponge is already active and may be watched 

 producing aqueous currents. At this age the sponge is an 

 excellent object for studying in the living condition: being 

 both small and transparent it affords us an opportunity of 

 watching the movements of particles of carmine as they are 

 carried by the current through the chambers. 



Potts ^ describes how he has followed the transportal of 

 spicules by dermal cells, the end of each spicule multiplying 

 the motion, swaying like an oscillating rod. 



In E. fluviatilis reproduction also occurs during the warmer 

 months in this climate by means of sexual larvae. These are 

 interesting for certain aberrant features in their metamorphosis.* 

 "While some of the flagellated chambers are formed in the normal 

 way from the flagellated cells of the larva, others arise each by 

 division of a single archaeocyte. This, it is suggested, is cor- 

 related with the acquisition of the method of reproduction by 

 gemmules, the peculiarities {i.e. development of organs from 

 archaeocytes) of which are appearing in the larvae. 



Definition. — We may now define sponges as multicellular, 



' Ann. Mag. Nat. Hist. (2), x. 1882, p. 365. 



2 P. Ac. Philad. 1887, pp. 158-278. 



' Evans, Quart. Journ. Micr. Sci. xlii. 1899, p. 363. 



