THE INVERTEBRATE COURSE. 121 



2. Arbacia punctulata. 



Two celled ! hr., 30 minutes. 



Four celled 2 hrs.. 30 minutes. 



Eight celled 2 hrs., 45 minutes. 



Sixteen celled 3 hrs., 45 minutes. 



Sixty-four celled 4 hrs., 15 minutes. 



Early blastula 7 hrs., 1 5 minutes. 



Late blastula 14 hours. 



Early gastrula 23 hrs., 30 minutes. 



Pluteus Two days. 



3. Echinarachnius parnia. 



Two celled i hr., 35 minutes. 



Four celled 2 hrs., 45 minutes. 



Sixteen celled 3 hrs., 15 minutes. 



Sixty-four celled 4 hrs., 1 5 minutes. 



Early blastula 6 hrs., 1 5 minutes. 



Late blastula 10 hours. 



Early gastrula 21 hours. 



Late gastrula 38 hours. 



Pluteus Two days. 



III. 



DISSECTION OF AMPHITRITE ORNATA : DR. E. F. ADOLPH. 



From a doped or freshly killed specimen obtain some of the 

 coelomic fluid by making a small slit in the body wall. What is 

 the color and consistency of this fluid? Examine under the mi- 

 croscope and observe the varieties of cells present and their move- 

 ments. Some of them are reproductive products. 



Pin down the specimen, either fresh or preserved, under sea 

 water with the dorsal or bristle-bearing surface upward. With 

 the scissors cut along the mid-dorsal line, piercing only the body 

 wall. Pin the body wall out flat as you cut from head to tail. 



Cccloin. Are there transverse septa or diaphragms? Note 

 the dorsal and ventral mesenteric strands which hold the diges- 

 tive organs in place. Cut the dorsal ones. How do the details of 

 internal segmental organization correspond to the external or- 

 ganization ? 



Circulatory System. This must be observed before further 

 dissection. Find the large, paired, sub-intestinal vessels. Trace 

 them backward as far as you can and forward into the single gas- 



