

209 





[work from thr port krin biological station.] 



NOTE ON A TETRAMEROUS SPECIMEN 

 OF ECHINUS ESCULENTUS. 



By H. C. Chad wick. 



Curator of the Biological Station, Port Erin. 



With Plate XVII. 



[Read May 13th, 1898.] 



The subject of this note was dredged by Professor 

 Herdman in Port Erin Bay, during Easter week, 1898, 

 and lay with several other normal specimens in the 

 Laboratory of the Biological Station until the following 

 day, when its abnormal character was fortunately noticed, 

 and it was laid aside for careful examination. 



The test measured 5 cm. in diameter, and was com- 

 posed of four ambulacra alternating with four inter- 

 ambulacra, one of which was slightly wider than the 

 rest. The apical system (PL XVII. , fig. 5) consisted of 

 four genital and four ocular plates. One of the latter was 

 much larger than its fellows, and apparently occupied 

 the position of a genital (o', fig. 5) in the system, though 

 it abutted externally upon the summit of one of the 

 ambulacra, and its pore was minute, like those of the 

 other oculars. The normal number of five pairs of 

 peristomial plates with their tube-feet,* and five pairs of 

 tegumentary gills were borne by the peristome (PI. XVII., 

 fig. 1), and in addition to these, one pair of tube-feet 

 (tf. r , fig. 1) which, as will be presently shown, probably 



* In " Forms of Animal Life," second edition, p. 560, it is stated that 

 "The feet belonging to the buccal plates of the peristome end not in a disc 

 but in two or more processes." This is not correct if applied to E. csculentus, 

 in which the peristomial tube-feet do end in a disc supported by a calcareous 

 rosette similar to those of the ordinary tube-feet of the test. 



