I06 AN INTRODUCTION TO THE STUDY OF FOSSILS 



OS. 



outer surface. The rigid skeleton is probably due to the irreg- 

 ular mesh-like union of the spicules. 



1. Sketch entire specimen, labeling osculum, incurrent canals. 



2. How would you know this to be a sponge ? 



Order j, Monactinellida. — Skeleton of one-rayed siliceous 

 spicules (whence the name from Greek monos, one, + aktis, 



ray, + Latin dim. ella). When these 

 spicules are united with one another 

 it is through the addition of spongin. 

 This order includes the few fresh- 

 water sponges (Spongilla, etc.) and 

 the majority of existing marine 

 sponges ; they inhabit the more 

 moderate depths. They are known 

 from the lower Paleozoic to the 

 present. 



The boring-sponges (Cliona) are 

 compound forms with many oscula. 

 They secrete pin-shaped siliceous 

 spicules encased in spongin, with 

 which, aided by a power of absorption, they bore passages in 

 dead or living shells for protection, not for food. These sponges 

 and boring worms help to perform the same function in the 

 economy of nature in the sea as fungi and insects do upon land, 

 namely the disintegration of dead organisms. 



Order 4, Ceratospongida. — Skeleton of horny, spongin fibers 

 (whence the name from Greek ceras, horn, + spongia, sponge). 

 Not definitely known in the fossil state. The common bath 

 sponge (Euspongia) is an abundant living form. 



The common commercial sponges grow in warm seas, as those 

 of the West Indies and the Mediterranean, from tide level to a 

 depth of 200 feet. When prepared for commerce they are torn 

 from the sea-bottom, exposed on the hot beach to decompose, 

 then washed in water to remove all fleshy parts, leaving the tough, 



Fig. 38. — The sponge, Astylo- 

 spongia prcemorsa Goldfuss, 

 from the Middle Silurian of 

 Indiana. X 2. Side view 

 showing the osculum {os.) 

 above. (After Hall.) 



