METAZOA PORIFERA. . 87 



connecting fibers are not easily determined. The two 

 specimens show variation in form. 



Carteriospongia radiata Hyatt, var. dulcina (No. 94), 

 is one of the most beautiful of horny sponges. It grows 

 upward from a stem in the form of delicate fronds. The 

 surface of the fronds is smooth and the fibers are so 

 closely woven that they form a veil on the upper side, 

 and sometimes on the lower, which bridges over the 

 inequalities of the interior. Large specimens of Carterio- 

 spongia may have as many as sixty branches. 



The most complex representatives of the group of 

 horny sponges belong to the family Spongidae. Nos. 95- 

 97 are Spongia tubulifera Lam., var. rotunda Hyatt. No. 

 95 is a vertical section through the body of the sponge, 

 showing the flesh, the large central tubes with radiating 

 tubes, and the openings of other tubes which run in all 

 directions through the sponge body. No. 96 is a dried 

 specimen of the flesh and skeleton, and No. 97 is the 

 skeleton with the flesh removed. The fibers are fine and 

 soft. No. 98 is another species of the same genus, S. 

 molissima Schm., in which the fiber's are dense and closely 

 woven. 



This collection of sponges with the supplementary 

 drawings illustrates the following points. 



The sponge animal arises from an egg which resem- 

 bles many adult Protozoa. 



The egg in its further development passes through a 

 blastula stage, thereby representing the adult Volvox of 

 the Mesozoa. 



The blastula stage is succeeded in most sponges by 

 a solid parenchymella stage. The endoderm arises by a 

 process either of immigration or of delamination of cells, 

 and a two layered organism is produced. Subsequently 

 an internal cavity and an external mouth opening are 

 formed. This stage is transient, since by the formation of a 

 middle layer or mesoderm the adult always becomes a 

 three layered organism. 



