136 INVERTEBRATE ZOOLOGY. 



supply little oval vesicles. thea/»^)»//(/e, of which the amhulacral feet are the 

 continuation. The entire apparatus forms a system by means of which water 

 may he forced into and out of the ambiilacral feet, causing them to be pushed 

 out and withdrawn. The relation of these parts is best understood through 

 the aid of cross-sections of a ray [113]. 



I I 2. Study of skeletal 2)lates. Cut off a ray and boil it in a test tube of 

 K-O-H until it falls apart. The little plates composing the skeleton are sep- 

 arated by this means. Wash several times by decantation and finally bring 

 the whole into a Stender dish for study. Compare these with another speci- 

 men, not boiled as much, and in which the plates still hold together. The 

 procedure can be regulated as desired by different degrees of dilution of the 

 reagent, and by varying the time of boiling. Obtain by these means an accu- 

 rate idea of the architecture of the skeletal plates, and express the result by 

 a diagram. 



I I 3. Cross-section of a ray. Select for this a few very small star-fish 

 and preserve by warm alcoholic sublimate. When sections are desired, re- 

 move a ray and decalcify by letting it stand several days (as long as necessary) 

 in a dish of picric acid, to which has been added a trace of nitric acid. The 

 acid should be daily changed and the process continued until the specimen is 

 no longer gritty when tested with a needle. Wash out in 70^, and stain 

 (2-3 days) in Alum Carmine. Dehydrate, imbed and section as usual. 



114. Larval forms. These are captured by the tow net. They are the 

 famous bipennariae and brachiolariae. They may be investigated as in the 

 case of other forms found in tow (naiiplias, etc.) and may be mounted either 

 stained or unstained. Handle with a pipette, or upon a slide. 



TYPE XXIII. -Arbacia punctulata. 



Strongylocentrotus Drobachiensis. 



I I 5. Occurrence and preservaiion. These are the " sea-urchins " common 

 on every coast. Arbacia is the small dark-purjjle form, found from Massa- 

 chusetts Bay southward, and Strongylocentrotus is the larger olive-green 

 species, with smaller spines, common farther north. Either species will do, the 

 minor differences making the comparison interesting. They may be preserved 

 as in the case of Asterias, but an artificial opening should be made into the 

 interior to allow the passage of the alcohol, as the skeleton is much firmer 

 than in the star-fish. A slit throiigh the peristome, along the edge of the test, 

 is the most practical. 



