102 AN INTRODUCTION TO THE STUDY OF FOSSILS 



incurrent, flagellate, and excurrent canals develop. The 

 osculum thus does not correspond to the gastrula opening, as it 

 does in the Coelenterata, but is a later development opposite it. 



1. Examine (a) an entire Grantia, (b) longitudinal and trans- 

 verse, sections under the compound microscope. 



2. Show by diagram how the animal eats. What does it eat ? 



3. How is the food digested? assimilated? 



4. How is respiration effected? excretion? 



5. How are the soft parts of the body supported and the 

 animal protected ? 



6. Are muscles present ? 



7. What is the nervous system like? 



8. Describe the reproduction and the development of an 

 animal from the egg to maturity. 



9. If the genus Grantia were to become extinct, what record 

 of its former existence and of its appearance would the rocks 

 preserve ? 



SUB-CLASS A, CALCAREA 



Skeleton of calcarous spicules (whence the name from Latin 

 calx, lime). Spongin not present. At present these sponges 

 occur in the shallower portions of the sea bordering the coasts ; 

 the fossil representatives (Middle Paleozoic to the present) Hved 

 under similar conditions, judging from the marly or sandy nature 

 of the strata inclosing them. 



Living examples are Grantia (described on page 99), and 

 Sycon. 



SUB-CLASS B, NON-CALCAREA 



Skeleton of siliceous spicules, of spongin fibers, or absent. 



This sub-class is divided into five orders. 



Order i, Hexactinellida. — Skeleton of six-rayed siliceous 

 spicules (whence the name from Greek hex, six, -f- aktis, a ray 

 H- Latin diminutive ella). When these spicules are united 

 with each other, it is through the addition of secondary silica, 

 never of spongin. To this order belong most of the fossil sponges. 

 At present they are characteristic of deep water, but the ancient 



