1 62 AN INTRODUCTION TO THE STUDY OF FOSSILS 



nerves through the center of the arm and pinnule plates and to 

 the stalk. A transverse section through an arm would thus 

 show a blood tube, a water tube and a nerve cord directly be- 

 neath the food groove, while deep within the calcareous plate 

 is lodged a second nerve cord. 



The sexes are separate. The dilated pinnule bases lodge the 

 ovaries and testes. 



The stalk is five-sided throughout in ancient fossil species 

 {e.g. P. asteriscus) but in living forms only near the calyx. New 

 plates are added directly beneath the calyx ; these are in living 

 Pentacrinus at first five-sided like the entire stalk of lower Meso- 

 zoic species ; but later they become round. These changes, in 

 which the animal recapitulates its ancestry, may be noted in a 

 single individual from the small plates directly beneath the calyx 

 to the large ones at the lower end of the stalk. The young stem 

 joints are more porous than older ones, and therefore not so apt 

 to be preserved fossil. 



The range of the genus is from the Jurassic to the present. 

 The symmetrically pentagonal stem joints of Pentacrinus as- 

 teriscus are very abundant in the Jurassic of the western part 

 of North x\merica. Living forms occur chiefly in the Caribbean 

 Sea and the Pacific Ocean. 



1. Sketch side view, noting calyx, arms, pinnules, stalk. 



2. Compare it to the starfish. 



3. How and what does Pentacrinus eat ? How breathe ? 



4. Of what does the nervous system consist ? 



5. Could the animal move ? Reasons. 



6. Is the skeleton external or internal ? Illustrate by means 

 of a plate of the stalk. 



7. What is the significance of the name crinoid ? 



8. What is the evolutionary significance of the fact that the 

 plates of the stalk immediately below the calyx are five-pointed, 

 but later become rounded ? 



Eucalyptocrinus (Fig. 63). Silurian, rarely Devonian. 



Calyx with a deep concavity at the stem end. Attached to 



the elongate ventral portion of the calyx and extending its full 



