EXTERNAL ANATOMY OF THE STARFISH 1 



1. Examine a preserved and injected Starfish and note (a) the star- 

 shaped, radially symmetrical body consisting of (6) a central disc from 

 which (c) five, pointed arms radiate. Note that (d) the convex upper, 

 or aboral, surface is markedly different from the flat under, or oral, sur- 

 face. Draw the specimen from the aboral surface, in outline, natural size. 



2. Note the circular madreporite embedded in the body wall of the 

 central disc between two of the arms. These two arms constitute the 

 bivium and the remaining three arms the trivium. A plane of sym- 

 metry which will divide the animal in right and left halves passes through 

 the madreporite, central disc, and the median longitudinal plane of the 

 middle arm of the trivium. 



3. Examine the aboral surface of your specimen under the dissecting 

 microscope and note that the external surface of the body wall is thickly 

 studded with blunt calcareous spines, around the bases of which, as well 

 as in the areas between, pincer-like pedicellariae will be found in great 

 numbers. Also numerous soft tubular branchiae, which function in 

 respiration, will be seen projecting through the body wall. These are 

 particularly well seen in an injected specimen. Add the details observed 

 in paragraphs 2 and 3 to your outline drawing. 



4. Examine the oral surface of your specimen, noting (a) the five 

 ambulacral grooves radiating from the central disc to the tip of each 

 arm and filled with (b) numerous projecting tube feet, each of which ends 

 in (c) a suction disc Note further (d) the mouth in the center of the 

 disc protected by (e) a series of large spines developed at the base of each 

 ambulacral groove. Draw the oral surface of your specimen in outline and 

 fill in the detail in one arm as observed. 



5. Examine the oral surface of a dried specimen and note (a) the 

 arrangement of the spines along the edges of the ambulacral grooves and 

 (b) the arrangement of the ambulacral ossicles forming the roof of the 

 grooves, through which the tube feet project. Draw a portion of one arm 

 to show the structure as observed. 



6. For comparison, examine a number of other common species of 

 Echinoderms, such, for example, as the Brittle Star (Ophiura), Sea Urchin 

 (Arbacia), Sand Dollar (Echinarachnius) , and Sea Cucumber (Cucumaria). 

 (W. f. 47.) 



1 B. pp. 74-76. 

 275 



