Chap. 22 SPONGES — a side line of evolution 459 



This cell divides into two as the process continues and when the spicule is com- 

 plete both cells move away. Spicules vary in shapes and sizes according to the 

 species. In general, they are elaborations of the single needle form. The most 

 beautiful spicules are the silicious ones composed of opal, a form of hydrated 

 silica. They are present, not only in the deep-sea glass sponges, but in the 

 fresh-water sponges, several of them very common (Figs. 22.6, 22.7). Bath 

 sponges contain interjoined fibers of spongin, a protein similar to that in hair 

 and feathers. The skeletons form an important basis for the classification of 

 sponges. 



Reproduction. Sponges reproduce sexually as well as asexually. In sexual 

 reproduction, female cells are produced in one individual and male cells in 

 another. Both kinds develop in the mesenchyme from especially large ameboid 

 cells. The sperm cells enter other sponges, whether male or female, by way 

 of water currents, and in the females the eggs are fertilized in the locations 

 where the embryos develop. In Sycon (older name, Grantia) the egg takes 

 in food, enlarges and protrudes into a cavity lined with collared cells pushing 

 some of the food with it. During the breeding season the large numbers of 

 sperm cells freed from male sponges in a vicinity make it inevitable that many 

 of them are carried through the incurrent pores of sponges whether male or 

 female. When they are brought into the female they enter the collared cells 

 that are adjacent to the ripe eggs. In the meantime one or more of the cells 

 loses its collar and flagellum, becomes ameboid and applies itself to the sur- 



16 cells 



48 cells 



hatching 

 stage 



amphiblastula inversion 



free-swimming 



fixation 

 (seen in section) 



Fig. 22.5. Development of a calcareous sponge, Sycandra: the ovum fertilized by 

 sperm from another sponge; the early embryo, 8, 16, and 48 cells, which is em- 

 bedded in the jellied middle layer (mesenchyme) of the parent's body wall; an 

 opening formed on the underside of the 48-celled stage functions as a mouth for 

 the embryo; blastula and beginning of hatching when the embryo makes its way 

 into the water passages of the parent; collared cells are already formed with flagella 

 extending into the blastocoel; amphiblastula: the embryo turns inside out by way 

 of an opening that first appeared in the 8-celled stage; the future upper end is up; 

 inversion; the future excurrent opening (osculum) is down; the larva is floating 

 in open water; fixation, compare Fig. 22.2. (After Schulze. Courtesy, Storer: Gen- 

 eral Zoology, ed. 2. New York, McGraw-Hill Book Co., 1951.) 



