Some of the Lozver Metazoans 379 



of small openings, the prosopyles. From the radial canals the water 

 goes to the spongocoel and out the osculum. This syconoid type of 

 construction is encountered more frequently than is the asconoid type. 

 The most common, however, is also the most complex, and is known as 

 the leuconoid type. 



The common bath sponge, Spongia, illustrates the leuconoid type 

 of construction. In this, the choanocyte layer is outfolded numerous 

 times, resulting in many small oval chambers which in turn may further 

 outfold. The choanocytes are found only in these chambers while the 

 spongocoel is usually obliterated by the mesenchyme which fills the 

 spaces surrounding the choanocyte-lined chambers. The outer surface 

 of the sponge contains many dermal pores which may open into large 

 spaces or directly into incurrent canals. From the canals or spaces are 

 passageways, prosopyles, by which the water passes into the flagellated 

 chambers. Other pores, the apopyles, open from the flagellated chambers 

 into the excurrent canals which unite with others to form a modified 

 osculum. This construction is such that an irregular, asymmetrical 

 mass is produced ; the size, of course, is usually much greater than that 

 attained by either of the other two types. While it was comparatively 

 simple to identify the individual sponge in the asconoid type of construc- 

 tion, it is nearly impossible among the leuconoid types. Theoretically 

 a single osculum represents a single individual, but delimiting the indi- 

 vidual in such a large mass is nearly impossible. 



Life History. — Asexual reproduction is very common among the 

 sponges. Currents of water often carry broken pieces to other sites 

 where they soon attach to form new colonies. By budding, colonies are 

 built up and increased in size. Internal buds or geinmules consisting of 

 masses of amoebocytes surrounded by spicules are often carried out 

 by the ever-present currents of water. 



During sexual reproduction, the ovum is fertilized in its position 

 in the mesenchymal tissues. The early holoblastic cleavages are simi- 

 lar to those of other animals. At the 16-cell stage, there are 8 large 

 cells at one pole and 8 small ones at the other. The large cells are 

 destined to become the epidermis, and the small ones the choanocytes. 

 These are more or less arranged as a flattened blastula. The small cells 

 increase rapidly and each acquires a flagellum on the end facing into 

 the blastocoel. The large cells round up and form an opening in the 

 middle which functions as a mouth to ingest cells from the mesenchyme 

 of the parent. Gradually this very modified blastula turns inside out 



