1908.] AN ABNORMAL ECHINUS. 655 



can assume that x is appi-oximately the same for the various 

 ambulacral plate-rows of the same specimen at any latitude, it is 

 virtually a constant for a particular latitude and therefore cannot 

 affect our comparison. 



In this Shetland specimen the development, up to a certain 

 stage, appears to have been normal, Thus on the oral surface 

 not only is the arrangement of the various rays regular, but 

 the line of bilateral symmetry is straight ; while in the minute 

 structure of the plates no abnormalities occur, save the single 

 insignificant deviation on area II b, where, although the plates 

 are normal in number and in arrangement, a suture is missing 

 between two triads. We are also justified in stating that during 

 the earlier stages of growth the ocular plate opposite the abnormal 

 •ambulacrum was perforated by an ocular pore, and that this pore 

 was occupied by the terminal tentacle of the radial water- vascular 

 system, for in no other way can the presence of the pin-hole 

 already mentioned be explained, seeing that in the ordinary 

 course of development the very existence of the pore is due to 

 the presence of the terminal tentacle *. 



At a certain stage, when rather more than twenty-eight (28 -fa:) 

 ambulacral plates had been formed, or, judging from young speci- 

 mens with a similar number of plates, when the test was between 

 20 and 25 mm. in diameter, some functional derangement took 

 place. As an immediate consequence ambulacrum V ceased to 

 grow, no more plates being added to that area after the thirtieth. 

 But a more general disturbance also occurred, for in each of the 

 rows of the five ambulacra abnormal plates were formed ; and in 

 these groups of aberrations, containing sometimes a sequence of 

 as many as five peculiar plates, the first abnormal plate, as a 

 glance at Table III. will show, is the twenty-eighth or the 

 twenty-ninth or, in a solitary case, the thirtieth. This approxi- 

 mation of numbers indicates, as we have already shown, that the 

 plates were formed approximately at the same stage of develop- 

 ment ; and the significance of the close numerical correspondence 

 between the commencing points of the abnormal series is not 

 lessened when we consider the difiiculty of counting the number of 

 plates at the edge of the peristome, and the uncertainty as to the 

 relative numbers that have been pushed off during development. 

 In themselves, considered separately, the abnormalities described 

 are perhaps of little significance, although we have been unable 

 to find, from examination of other tests, that such abnormalities 

 are of frequent occurrence. Bvit that abnormalities so distinct 

 should manifest themselves at all points of the test at practically 

 the same period is indeed remarkable. There can be but one 

 explanation, namely, that a general derangement afi'ecting all 



* " [Die Primordialtentakel] werclen endlicli von den sicli bildendeu Radial- 

 (Ocellar-)Platten dereu Rand sie beriiliren, umwachsen xind sind so zu den 

 Terminal- oder Endfiihlem geworden. . . . Sie treten dann durch einen Porus der 

 Platte hindurch." Hamann, Otto, " Die Ecliinodermen " ; Bronu's Klassen und 

 Ordnungen des Thier-Reiehs, ii. Bd., 3 Abth, p. 1167. 



Proc. Zool. See— 1908, No. XLII. 42 



