282 THE AMERICAN NATURALIST [Vol. LVI 



tween adjacent ambulacral ossicles. This, tlie common arrange- 

 ment, may be designated Type I. It will be noted that as one 

 runs along the arm the transverse canals of succeeding pairs 

 are long-short, short-long, and so on. 



However, it appears from my experience in this laboratory 

 that some teachers give a different description of the arrange- 

 ment of the tube-feet. According to these teachers the length of 

 the transverse canals does not alternate in a single pair, but is 

 the same on both sides of the radial canal. This would lead to 

 an arrangement which is shown diagrammatically in Fig. 2, 

 According to this account as one runs along the arm the trans- 

 verse canals of succeeding pairs are long-long, short-short, and 

 so on. It seemed worth while from a teaching standpoint to de- 

 termine which of these descriptions is the correct one. For con- 

 venience, the arms of the starfish will be named in the conven- 

 tional manner a, b, c, d, e—a being the first arm to the right of 

 the madreporic plate (as seen from the aboral surface), the 

 others being named in a clock-wise direction around the disc. 



In all, seventeen specimens of Asterias forhesi have been ex- 

 amined, some more completely than others. The first two or 

 three pairs of tube-feet at the very base of the arm are usually 

 rather crowded by the abrupt narrowing of the ambulacral 

 groove, so that it is rather difficult to say exactly to which type 



