PHYLUM PORIFERA 99 



Spongilla and several marine sponges have a peculiar method 

 of reproduction by the formation of gemmules. A number of 

 germinal cells in the middle layer of the body-wall gather into 

 a ball and become surrounded by protecting spicules. These 

 gemmules (Fig. 58) are formed in the autumn just before the 

 death of the adult sponge. In the spring they develop into new 

 sponges. They are of value in carrying the race through a 

 period of adverse conditions, such as the winter season. 



4. SPONGES IN GENERAL 



(i) Morphology. EXTERNAL FEATURES. - - Leucosolenia, 

 Grantia, and Spongilla have served as types of the Phylum PORI- 

 FERA, but other sponges vary markedly from these both in form 

 and in structure. In many cases the character of the object to 

 which sponges are attached causes them to assume exceedingly 

 irregular shapes, the rocks being frequently incrusted by in- 

 definite masses of spongy tissue. The habit of growth of many 

 sponges is responsible for their shape. Some are branched like 

 trees, or form a network; others are fan-shaped, cup-shaped, 

 or dome-shaped. Some sponges are no larger than a pinhead; 

 others are over five feet high. Most calcareous sponges are 

 white or gray, but others may be brilliantly colored and even 

 iridescent, exhibiting all the hues of the rainbow. 



CANAL SYSTEMS. There are three principal types of canal 

 systems exhibited by sponges: (i) ascon, (2) sycon, and (3) 

 rhagon. That of Leucosolenia (p. 94, and Fig. 59, A) is of the 

 ascon type, and that of Grantia (page 95, and Fig. 59, B) is of the 

 sycon type. Some sponges, like Spongilla, have a very compli- 

 cated canal system; this, the rhagon type, is diagrammatically 

 shown in Figure 59, C, and described on page 98. 



SKELETAL SYSTEMS. The skeletons of sponges are composed 

 of spicules of carbonate of lime or silicon, or of fibers of spongin. 

 A few small species have no skeletons. Some of the more com- 

 mon types of spicules are shown in Figure 60; they are (i) 

 monaxon (a, b),,(z)ltftr&0n> (c, d), (3) iriaxon (e^Cand (4) poly- 



