INVERTEBRATE ZOOLOGY. 137 



116. E.rtcrnal anatomy. Study and explain the symmetry of the ani- 

 mal. What is the shape of the shell 'i Find the oral and ab-oral aspects. 

 Oral side : — In the center, the circular lij). siirrounding five teeth, set in hard 

 jaws. The soft area continiious with the lip is the 2>e7'istome, reaching to the 

 margin of the shell or test. Upon the peristome are ten large amlnilae)-al ten- 

 tacles, surrounded by pedicellariae. At the border of the test are the oral 

 ends of five rows of ambulacral feet. Trace these around upon the ah-oral 

 side. Ah-oral side : — In the center, the anus protected by four anal plates (in 

 Strougylocenti'otus the four plates are replaced bj- numerous little oval 

 plates). Surrounding the anal plates are five large genital plates, alternating 

 with five small <>ceZ/arp/aie.s'. One of the genital plates includes the madre- 

 poric area. Find the genital openings and the eye spots in their respective 

 plates. What relation do these plates sustain to the rows of ambulacral feet ? 



Spines and test : — Notice the spines. Tear one away from the test and no- 

 tice the articulation. Have the spines any regular arrangement':' Can the 

 position and size of the spines be determined on a denuded test V Beginning 

 at the anus, pull off the spine from a considerable area, inchiding a row of 

 ambulacral feet and the entire space between two rows. Notice that the po- 

 sition of the ambulacral feet is indicated by two rows of perforated plates, 

 while the space between two ambulacral rows is filled bj' two rows of larger 

 plates. The area of the feet is termed the ambulacral area, and the space be- 

 tween them the i nter-ambidacral area. Which bears the largest spines? What 

 is the arrangement of the ambulacral feet (as determined l)y the ambulacral 

 pores)? What is the total niimber of rows of plates running meridionally 

 around the test? [After the dissection, the test may be cleaned by bt>iling in 

 dilute K-O-H, and the separate plates isolated. If a perfect test be taken, tlie 

 separate skeletal elements may be mounted on a black card as in the diagram 

 of Lang]. 



117. Dissection. This should be performed from the oral side. Remove 

 the peristome and crack away the margin of the test as far as desired. In 

 the centre lie the teeth and .jaws, forming a sj^mmetrical cone-shaped mass. 

 ''Aristotle's lantern."' Lift this carefully and notice that the alimentary 

 canal runs through it, the free part beginning at the center of its base. Se^ja- 

 rate and study the parts of the " lantern." It is moved by muscles attached 

 to the edge of the test, and from one part to another of the lantern itself. 

 The internal anatomy is similar to that of Asterias. presenting, however, 

 interesting minor differences in every system. Dissect it carefully and com- 

 pare step by step. 



