SHOETEE AETICLES AND DISCUSSION 



A TEACHING NOTE ON THE ARRANGEMENT OF THE 

 TUBE-FEET IN ASTERIAS 



Several summers ago while directing the laboratory work on 

 Echinodermata in the Invertebrate Course at Woods Hole, a 

 question was raised by one of the students as to the correctness 

 of the account which had been given of the arrangement of 

 the tube-feet in the common starfish, Asterms forhesi Desor, 

 An examination of the point in question revealed the occurrence 

 of a rather interesting irregularity which is here briefly re- 

 ported on. The source of the conditions here described is 

 entirely unknown, but inasmuch as this starfish is commonly 

 used as material for laboratory study, it occurred to me that 

 the facts themselves might be of interest to teachers of inverte- 

 brate zoology. 



It will be recalled that in Asterias the tube-feet are arranged 

 in four longitudinal rows, two of which are on each side of the 

 radial canal (the mid-ventral line of the arm). The tube-feet 

 in these rows of two are arranged in an alternate manner, nearer 

 or farther from the mid-line, thus allowing for the accommoda- 

 tion of more tube-feet in a given linear space. The tube-feet 

 are connected with the radial canal by short transverse canals, 

 which are thus longer or shorter according as they pass to tube- 

 feet in the inner or outer series. This arrangement of the tube- 

 feet can be clearly made out in properly dried specimens from 

 which the remnants of the tube-feet themselves are all removed. 

 Their position is clearly marked in such a preparation by the 

 perforations between each pair of ambulacral ossicles, and the 

 whole topography of the ambulacral groove is well demonstrated. 

 It is usually stated that ^ "Each pair of transverse canals con- 

 sists of a short canal on one side and a longer canal on the 

 opposite side of the radial canal. The short and long canals of 

 each side are alternating." This arrangement of the tube-feet is 

 shown in a diagrammatic way in Fig. 1, in which the tube-feet 

 are represented as black ovals situated in the perforations be- 



1 Quoted from Petrunkevitch, " Morphology of Invertebrate Types," New 

 York, 1916, pages 177 and 178. 



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