CANAL SYSTEMS OF SPONGES. 15 



mouths or holes on its surface, by which it sucks in and throws out 

 the water." 



Now to trace something of the progress that has been made in 

 the study of these animals during the last hundred years. By most 

 authorities, sponges are granted a separate phylum or branch, in the 

 genealogical tree of the division Metazoa or multicellular animals. 

 They form the lowest or least specialized phylum ; the name applied 

 to them collectively being Porifera, A short summary of the most 

 important points in the anatomy of the chief types of sponges is 

 however necessary to the intelligent understanding of such position 

 in the scale of nature. 



The most primitive form of sponge organization is well seen in 

 the very simple sponge, Ascetta primordialis, so minutely described 

 by Haeckel. In appearance, Ascetta is a tiny, thin-walled, goblet- 

 shaped animal that lives anchored by the narrow stalk end to rocks 

 and weeds. At the free end is a wide opening (PI. iv. Fig. 1), the 

 OSClllum, while minute holes, pores, pierce everywhere through the 

 thin walls and open into the great central cavity or paragaster. These 

 walls, thin as they are, are of considerable complication, for three layers 

 of cells can be made out, an external, ectoderm, composed of a single 

 thickness of more or less mosaic-like cells ; a middle, mesoderm, 

 here extremely thin, and lastly an internal layer, endoderm, 

 composed in this sponge of oval shaped cells, each provided at the 

 free end with a well-marked circular upstanding collar, from within 

 the centre of which a strong whip-like thread or flagellum arises. 

 Such cells are termed flagellated collar cells or shortly, flagellated 

 cells technically choanocytes. 



In the first four figures on PI. iv, the outer black line repre- 

 sents both ectoderm and mesoderm, while the shaded layer stands 

 for flagellated cells. In the living sponge, the activity of the flagella 

 of the endoderm sets up minute currents of water flowing through 

 the numberless pores into the paragaster where nutrient particles 

 are picked up from the water and effete matter cast back. Hence 

 these currents, now gathered into a strong body of water, are directed 

 out through the great osculum at the summit of the sponge. Sponge 

 circulation is always fundamentally similar, even in the most complex : 

 ingress by very small openings, egress by a single large vent. 



But to return to our Ascetta. Essentially its structure is a 

 thin walled sac, with pores opening direct into a great paragaster 

 lined with flagellated cells. This arrangement of water passages, 

 technically Canal System, is the simplest known, constituting what 

 Ha3ckel named the primitive Ascon Type (Fig. 1). 



Complications are frequent, but the fundamental feature of 

 flagellated paragaster remains stable. Thus in our common Ascetta 



