236 



KNOWLEDGE 



[December 1, 1891. 



peculiarities of these creatures, which will form stepping- 

 stones for those inclined to proceed fm-ther with their 

 study. 



To begin with, Sea- Urchins take their name from the 

 array of movable spines with which the shell is covered 

 during life, and which thus suggest comparison with those 

 of the true Urchin, or Hedgehog. The shell, as it is 

 commonly called, will, indeed, be the portion of . the 

 animal to which alone oiu' attention will be directed, since 

 it is only this part that is capable of preservation in a 

 fossU state. We must, however, state at the outset that, 

 as this so-called shell does not by any means correspond 

 in structure with the shell of a mollusc, it is found more 

 convenient to give it a different name, and the term test 

 has accordingly been selected. Moreover, since the name 

 Sea-Urchin is a somewhat long one, we may conveniently 

 abbreviate it to Urchin, unless we prefer to use the more 

 technical term Echinoid. 



There is great variety in the form of the hard calcareous 

 test of the Urchins, which varies from a shape somewhat 

 resemblmg a flattened orange to a heart-shape, or even to 

 a thin disc-like plate. The ordinary Urchins shaped 

 somewhat like an orange are, however, those best adapted 

 for gaining a general idea of the structure of the group, 

 and we shall accordingly commence with them. 



If, then, we examine such a test, we shall find that it 

 has an aperture at each of the two poles ; while it is 

 divided into a series of meridional areas, each composed of 

 a number of separate oblong calcareous plates, fitting 

 accurately with one another at their edges, where they are 

 united by a thin membrane. The upper surface of such a 

 test is shown in Fig. 1, from which it will be seen that 

 the upper or apical pole, 

 as it maybe conveniently 

 called, consists of five 

 somewhat heart-shaped 

 plates siUTounding a 

 small circular orifice 

 which is closed with 

 membrane in the living 

 state. This central ori- 

 fice is the fi'nt, while 

 the five plates forming 

 a star surrounding it, 

 which generally fall 

 out in the dry state, 

 constitute the apical disc. 

 At the opposite or lower 

 pole, we shall find a 

 much larger aperture, 

 which forms the crea- 

 tm-e's mouth ; and we 

 may observe that just inside this aperture of the test in 

 the complete animal, there \^"iU be found a very compli- 

 cated calcareous masticating apparatus kno'^Ti as Aristotle's 

 lantern. Putting aside for the present the apical disc, we 

 may devote somewhat fuller attention to the main body of 

 the test, teohnicall}' termed the corona. As we have said, 

 this consists of a series of meridional areas composed of 

 numerous small plates ; and we shall find that these areas 

 form ten alternating series, each of which consists of two 

 meridional rows of the aforesaid plates. The line of 

 division between the two rows iu each area is well marked, 

 although the divisions between any two areas are much less 

 distinct. It will further be seen that, while in one series 

 of areas (( a) the plates are much wider than in the other 

 series [a), they are also somewhat deeper. Now, if we 

 were to look from the inside of the test towards the light, 

 we should find that the outer half of each plate in the 



Fio. 1. — Upper Surface of the Test of 

 the Cominon Sea-Urcliin. a. Am- 

 biUacral areas j i a. intermediate 

 areas. 



narrower areas has several minute perforations ; and it is 

 through these minute perforations that, during life, the 

 animal protrudes the curious tube-like feet, by the sucker- 

 like action of which an Urchin is enabled to climb up the 

 glass wall of an aquarium. As these narrower areas are 

 connected with the function of progression, they are 

 appropriately termed the amlndacral areas, and the larger 

 intervening spaces are accordingly called the intermeiUate 

 areas. It is almost superfluous to add that the whole test 

 of an Urchin thus consists of five ambiilacral and five 

 intermediate ai'eas, which between them comprise twenty 

 separate rows of plates, each running continuously from 

 one polar aperture to the other. During life each platei s 

 separated from its neighbour by a thin membrane, and 

 the test increases in size both by additions to the edge of 

 each plate and also by the interpolation of fresh equatorial 

 zones of plates between the upper edge of the corona and 

 the apical disc. The spines of the Urchins are movably 

 attached to the knobs with which the test is covered, and 

 vary much in form and size, although the limits of this 

 article do not admit of further reference to them. 



Reverting to Fig. 1 , we may also see that the ambiilacral 

 areas of the Common Urchin form a five-rayed star, on the 

 test of which, in the position of the figure, three rays are 

 turned away from the spectator and two towards him. 

 Now this radiate arrangement at once forciby reminds us 

 of a Star-Fish, and any person who has ever handled those 

 creatures when alive will be aware that from their under 

 surface they can protrude tube-like sucking feet, precisely 

 similar to those we have referred to as existing in the 

 Urchins. Both these points of resemblance are, indeed, 

 indicative of an intimate relationship between Urchins and 

 Star-Fish. And, as a matter of fact, the ambiilacral areas 

 of the one represent the five rays of the other ; the inter- 

 mediate areas of the Urchins being an addition to the 

 stritcture of the Star- Fish. At first sight it looks, indeed, 

 as if these animals were really symmetrically radiate, but 

 we shall show later on that this is not the case, and that 

 they are, in truth, bilaterally symmetrical like the higher 

 animals, although this bilateral symmetry has been more 

 or less thoroughly masked by the radiate arrangement of 

 the parts. 



Urchins and Star-Fishes are, however, not the only 

 members of the group of Echinoderms ; since this also 

 comprises the beautiful Stone-LiUes or Encrinites, the 

 joints of the stems of which form the well-known " St. 

 Cuthbert's Beads," of the Whitby Lias, while the so-called 

 Entrochal Marble, so often employed for chimney-pieces 

 and other decorative architecture, is almost entirely com- 

 posed of these stems. There are, moreover, certaLa 

 entu-ely extinct types, such as the Cystoids and Blastoids, 

 of which more anon. 



Before proceeding to trace the modifications which the 

 test of the Urchins undergoes iu different members of the 

 group, it may be observed that iu all Urchins, whether 

 recent or fossil, the number of meridional areas is in- 

 variably ten ; while in every existing kind of Urchin, no 

 matter what be its size or shape, the number of rows 

 of plates composing such areas never departs from twenty. 

 As soon, however, as we reach the strata Ijiug below 

 the chalk and gault, known as the lower greensand, which 

 constitute the lower part of the Cretaceous system, we find 

 an Urchin which departs somewhat in the last-named 

 respect fi-om the existing type. A side view of the test of 

 this species is given iu Fig. 2, from which it will be seen 

 that while at the apical pole the number of meridional 

 rows of plates is the normal twenty, as we approach the 

 equitor the number of meridionil rows in each inter- 

 mediate area is increased to four, which continue to the 



