THE STARFISH 139 



around the mouth at the margin of the peristome. Distally the radial 

 nerve ends in the eye-spot which can usually be seen by examining the 

 outer end of the ambulacral groove with a handlens. These portions 

 of the nervous system are in the epidermis. There are deeper portions 

 of the system beneath the epidermis of the oral surface which are not 

 visible in a gross dissection. These consist of paired radial nerve cords 

 attached to a double oral nerve ring. In addition there is an aboral 

 nerve cord located in the peritoneum in the mid-line of each arm and 

 connected with an anal nerve ring. Draw (XI) the parts of the 

 nervous system that you can identify. 



Exercise 8. — The Circulatory System and Ccelom. 



(i) The extensive coelom has been noted as a conspicuous feature 

 of the internal organization. There is no well-developed circulatory 

 system, but there is a system of cavities usually designated as the 

 blood system or haemal system. Against the outer surface of the 

 stone canal is a fleshy body, the axial sinus. If these two structures 

 are cut transversely in their middle region, and the cut surface ex- 

 amined under water with a handlens, the cavity of the axial sinus will 

 be clearly seen. What is the appearance of the stone canal in this 

 region? You may also be able to recognize the axial organ as a ridge 

 on the inner surface of the axial sinus. The axial sinus joins a canal 

 which encircles the mouth and from which vessels pass along the 

 aboral surfaces of the arms between the epidermal radial nerves and 

 the radial canal of the water-vascular system. Transverse sections of 

 an arm will often show this radial vessel and may also show that it is 

 divided by a vertical partition. Although the function of this canal 

 system is not definitely known, it is generally considered comparable 

 to that of a blood-vascular system. 



(j) Reexamine Fig. 63 in review of your study of the specimen. 

 How does the starfish illustrate the triploblastic body-plan, when 

 compared with the annelids and vertebrates? What are the essential 

 similarities? In what ways does the structure of the starfish seem 

 aberrant? 



m. REGENERATION, REPRODUCTION, AND DEVELOPMENT 



Exercise 9. — Regeneration. 



(a) Regenerating specimens may be studied alive, but where time 

 is limited not much can be observed that is not as well shown by a 

 properly selected series of dried specimens in various stages of re- 

 generation. How does the process of restoring the normal proportions 

 of the parts compare with that in a regenerating planarian? Examine 



