December 1, 1891.] 



KNOWLEDGE 



237 



basal pole. Going 

 still further back 

 to the Trias, 

 we find another 

 genus of Urchins 

 (Tiarerhinus) with 

 an increase in 

 the number of 

 rows of i^lates in 

 the intermediate 

 areas above the 

 lower pole. More- 

 over, some of the 



Fio. 2. — Side view of the test of a Cretaceous Urchins of the 



Sea-Crchin {Tetracidaris). Letters as in gtiH older .Jurassic 



^'e- 1- rocks differ from 



the ordinary type, in that the plates overlap one another 



instead of joining by their edges. 



These departures from the normal form in some of the 

 Secondary Urchins suggest that, if we were to go back to 

 the PalsBOZoio epoch, we should find still more marked 

 differences from living types. Such, indeed, is actually the 

 case, and we find that all the Palipozoic Urchins differ 

 from existing ones in the number of meridional rows of 

 plates, while very frequently these plates overlap one 

 another. In the specimen represented in Fig. 3 it will be 

 seen that while the ambulacral areas 

 ai'e normal, the intermediate areas 

 have now no less than five meridional 

 rows of plates ; while in yet another 

 form (Mclaniti'x) tlie ambulacral as well 

 as the intermediate rows are increased 

 in number, the former varying from 

 seven to eight, and the latter from 

 eight to fourteen. With one ex- 

 ception, however, all the Paleozoic 

 Urchins agree with the Common 

 Urchin in having the vent situated at 

 the apical, and the mouth at the 

 basal pole, this mouth always having 

 the " Aristotle's lantern." 



Now if we proceed still further back 

 till we reach the lower part of the Pale- 

 ozoic epoch, such as the Cambrian 

 and Lower Silurian, we shall find a totally extinct group 

 of Echinoderms known as Cystoids. The hard parts of 

 these creatures consist of a nearly globular test, usually 

 supported on a short stalk, and composed of a number of 

 polygonal plates, having no definite meridional arrange- 

 ment, but traversed by five, or fewer, irregular ambulacral 

 grooves radiating from the mouth. And it will be obvious 

 that, in their large number of meridional rows of plates, 

 the Pabi'ozoic Urchins present a much closer resemblance 

 to these extinct Cystoids than is offered by their existing 

 representatives. Indeed, if we believe m the derivation of 

 one form of animal from another, it seems pretty evident 

 that starting from tlie Cystoids — tlie oldest known Echino- 

 derms- we can pass readily into the Pabeozoic Urchins, 

 from wliich we are conducted by the above-mentioned 

 intermediate Secondary forms to species of the type of the 

 Common Urchin of the present day. What particular 

 advantage the modern Urchins have gained by the 

 reduction of their meridional rows to twenty is, however, 

 not very easy to determine ; although this reduction has 

 probably conduced to greater compactness and strength in 

 tlie structure of their test, which may alone have been a 

 sufficient improvement on the older types. 



The transition from the Pahvozoic to the modern forms 

 does not, however, by any means exhaust the modifica- 



lou. 

 Fio. 3. — Side view of 

 the test of a Palteo- 

 zoic Sea - Urchin 

 {Paleeoechiniis) . 

 Letters us in Fig. 1. 



LCt 

 Fig. 4. — Upper surface of the test 

 of a Cretaceous Heart Urchin 

 (Micraster). Letters as in Fig. 1. 



tions which the Urchins have imdergone with the march 

 of time. Pieverting once more to the Common Urchin 

 (Fig. 1), it should be mentioned that this tj'pe, in which 

 the test is radiately symmetrical and the vent and mouth 

 are polar, constitutes the group of the so-called Regular 

 Urchins. Although this type is still well represented, yet 

 a large number of the Urchins of the Secondary, Tertiary, 

 and recent periods have departed very considerably from 

 this simple form, to assume a more or less decided heart- 

 shape, with one or both apertures of the test becoming 

 eccentric, and with a frequent tendency for the perforated 

 portions of the ambulacral areas to become restricted to 

 the central part of the upper surface, where they form 

 a flower-like pattern (Fig. 4). Such types constitute 

 the group of Irregular 

 Urchins, which, on the 

 doctrine of evolution, 

 may be safely regarded 

 as derived from the Reg- 

 ular group. Moreover, 

 not satisfied with the 

 assumption of these new 

 shapes, the Irregular 

 Urchins appear to have 

 considered the " Aris- 

 totle's lantern," which 

 had served their ances- 

 tors as a masticating 

 organ for countless ages, 

 only a useless encum- 

 brance, and, accordingly, 

 the more advanced " radi- 

 cals " among them have 

 totally discarded this 

 piece of apparatus, and, indeed, appear to get on equally 

 well without it. 



The forms connecting the more aberrant members of 

 the Irregular with the Regular group of Urchins are both 

 numerous and varied. Among them we may refer to the 

 common little Helmet Urchin (t^cliiimomtis) of our chalk. 

 The test of this species is about an inch in height, and 

 forms a tall cone, which is not quite radiately symmetrical. 

 The mouth, indeed, retains its usual position at the basal 

 pole, while the perforated areas extend from pole to pole. 

 In place, however, of the vent being situated in the centre 

 of the apical disc, it has been transferred to the margin of 

 the lower surface, in the middle of one of the intermediate 

 areas. It is thus placed opposite to an ambulacral area 

 corresponding to the one directed upwards in Fig. 1 ; and 

 we thus, for the first time, find a regular bilateral sym- 

 metry fuUy established. In the particular species to wliich 

 we have referred (as in all the under-mentioned forms), 

 the " Aristotle's lantern " has been lost, although it is 

 retained in other members of the same group. One 

 specimen of a Helmet Urchin has been discovered show- 

 ing the rare abnormality of having but four ambulacral 

 areas. 



A further step is shown by the so-called Sugar Loaf 

 Urchin (AnKiichi/tfs) of the English chalk, the tall silici- 

 fied tests of which are so commonly found in the gravel 

 derived from the denudation of the chalk. In this species, 

 which is considerably larger and more ovoid than the 

 Helmet Urchin, the elevated form of the test is still 

 retained, but the contour of its under surface lias become 

 more decidedly heart-shaped. Moreover, the vent has 

 become shifted" completely on to the lower surface ; while 

 the mouth, although remaining (as is invariably the case) 

 on the under surface, has travelled away from its original 

 central position so as to be placed in the ambulacral area 



