SUMMARY OF IMPORTANT FACTS. 227 



seas) and the Tetractinellid^ : Geodia* Near here apparently belongs 

 Oscarella;* without a skeleton (Myxospongia). 



In the MoNACTiNELUD^ the spicules are united by spongin (Corna- 

 cuspongia), and can even be entirely replaced by that substance. Numer- 

 ous marine forms, among them Chalina* and also the fresh-water 

 SpongilliDjE {Hpongilla* Ephydatia *), widely distributed as encrusting 

 masses on submerged sticks and stones. The natural color is light gray, 

 but they are usually colored green by Alg;e. They arc distinguished 

 from most marine relatives by the formation of gemmuhe or statoblasts. 

 At times the protoplasm divides into round bodies, as large as the head of 

 a pin and these become surrounded by a firm membrane strengthened in 

 many forms by collar-button-like spicules, the amphidiscs. These stato- 

 blasts remain entangled in the skeleton and survive times of freezing or 

 drought. On return of good conditions the contents escape and form 

 small HponyillcR, often utilizing the old skeleton. This process recalls 

 encystment among the Protozoa. 



When the spicules entirely disappear and nothing but the spongin 

 fibres remain we have the horny sponges or Ceraospongle. The skeleton 

 consists of a horny substance, spongin, which differs chemically from the 

 substances of true horn — keratin. This spongin is always laid down in 

 long fibres by peculiar cells, the spongioblasts, and it always consists of 

 concentric layers. The fibres interlace, branch, and unite into a skeleton. 



The best known horny sponges are the bath sponges, Euspongia 

 offiairtali.s* occurring in the Mediterranean, West Indies, Florida, and 

 other seas in many varieties. Best of all are the Levant sponges (var. 

 ■mollissima). Sponges of commerce consist only of the skeleton, the ani- 

 mal parts being killed and, after decay, washed away with fresh water. 

 Less valuable are Euspongia zimocca and Hippospongia equina* the 

 horse-sponge, while the Cacospongia are useless. 



Summary of Important Facts. 



1. The sponge body is largely a mass of connective tissue cov- 

 ered externally with pavement epitlieliuin (meso-ectoderm) and 

 penetrated by canals. 



2. An entoderm of collared flagellate cells occurs only in the 

 ampullar or flagellate chambers which are intercalated between 

 incurrent and excurrent canals (in ascons in the central cavity). 



3. The animals receive nourishment through fine pores in the 

 body wall; indigestible bodies are cast out through one or several 

 oscula. 



4. .Since nerves, muscles, and sense organs are lacking or very 

 weakly developed, the animals show tlie most inconspicuous move- 

 ments. 



5. Sponges are divided into Calcispongiaj and Silicispongiaj 

 according to the character of the skeleton. 



