NOTE OX TETRAMEROUS SPECIMEN OF ECHINUS. 289 



represented those of the absent ambulacrum. Each of 

 these tube-feet was seated upon a minute ossicle (os., 

 fig. 3), through the centre of which ran a perforation. 

 Upon the distal margin of the ossicle, and encircling the 

 tube-foot, spines and pedicellaria were seated 167;, ped., 

 fig. 2). In the semi-contracted condition 111 which it was 

 examined the ossicle and its tube-foot together measured 

 3 mm. in length. 



The test being opened by a meridional incision, the 

 intestine was found to follow the normal course, and 

 four well-developed ovaries occupied the interarnbulacral 

 areas. The jaw apparatus, or ''''Aristotle's lantern,'' was 

 quite normal, all its parts numbering five. But, there 

 being only four ambulacra, two of the five pairs of adductor 

 muscles of the teeth were attached to the margin of one 

 interambulacrum (amt., fig. 4), and, between the two pairs, 

 a feebly developed pair of opening muscles of the teeth 

 were also attached. Each ambulacrum bore a well- 

 developed auricula. The circum-oral water vessel bore 

 five Polian vesicles, and five radial vessels proceeded from 

 it. Of these latter four traversed the corresponding 

 ambulacra to the apex of the test in the normal way, 

 while the fifth passed through the peristome near its 

 margin, and ended in the pair of tube-feet described 

 above. This anatomical relationship points to the view 

 suggested above, i.e., that the tube-feet represented those 

 of the absent ambulacrum, upon which it follows that 

 the perforated ossicles upon which they were seated were 

 the feeble and displaced representatives of the plates of 

 that ambulacrum. 



Assuming that the madreporite occupied its normal 

 position on the right anterior genital plate (genital 2 

 according to Eoverrs formula), it is evident that the 

 genital marked * in fig. 5, occupied the position of the 



