22 



PROP. AGASSIZ'S 



there are as many large ones as there are large 

 plates. The first of these p'.ates,which are solidified 

 in Sea-Urchins, are those which surround the 

 rnouth, and which form the outline of that opening 

 which is closed by a membrane,and in the centre of 

 which we realk find the mouth in the perfect an- 

 imal. Above is formed the upper disc, consisting 

 chiefly of the plates, alternately larger and smaller, 

 through five of which the ovaries are discharged, 

 minute eyes being placed in the five others. 



The new plates of the sphere are gradually form- 

 ed above the older ones, around the mouth, and 

 wherever additional plates are developed, they 

 arise higher and higher. In this way we see that 

 the young sea-urchins the young Echini are 

 growing larger by the addition of plates between 

 those of the upper disc and those which surround 

 the mouth. 



/ 



But what are those plates of the upper disc ? 

 The five smaller contain the eyes and stand above 

 the rows with pierced plates ; the five larger ones 

 give passage to the ovaries and stand above the 

 rows with imperforated plates. 



Now in star-fishes we have similar eyes, colored 

 dots, at the extremity of the rays, (Plate IV. fig. A) 

 The plates which protect them (Plate IV. fig. B) 

 correspond therefore to the smaller perforated 

 plates, of the upper disc of Echini, (Plate XVIII.) 

 and the ovarial plates correspond to the angles be 

 tween the rays of star-fishes. 



(PLATE XVIII-SUPERIOR DlSC OF SEA URCHINS.) 



It is therefore no exaggeration when we say that 

 a star fish is a sphere stretched into a pentagonal 

 shape, and in which the eyes are carried out into 

 the rays ; as there are holes opening between the 

 rays in their angles where the ovaries open. In 

 this, as well as in every other respect, the analogy 

 is most complete. Vice versa, we may say, that 

 Echini are swollen or spherical star-fishes, with 

 reduced rays ; and Holothuriae animals of the same 

 structure drawn out into a worm-like tubular form. 

 It is really of some importance to be able to trace 

 this comparison in detail, as it will now at once 

 enable us to show that the analogy of the various 

 embryonic forms with the perfect animals is made 

 out sufficiently to afford the means of appreciating 



their relative positions in a natural system, lyy the 

 analogies which exist between the full grown ani" 

 mals of this class and the changes which they un- 

 dergo in their formation. Not only are the plates- 

 increasing and the body enlarging, but also its form 

 is assuming peculiar modifications. From these 

 pentagonal forms it is transformed into a regular 

 star, The Sea-urchin with a flat disc, as we have 

 it here (Plate XI fig. G.) is transformed into a 

 spherical body, seen here. (Plate XIII fig. B.) 



[PLATE VIII STAR FISHER 1 



The star-fish is also gradually transformed from 

 its outlines in Plate IV, into the perfect animal, 

 (Plate VIII.) It now becomes an important point 

 to be able to ascertain to what peculiar forms of 

 Sea-urchin those embryos belong,as we have among 

 the living ones some with the flattened disks, oth- 

 ers with a spherical form, and others with more 

 prominent elongated forms. Let us see what sort 

 of living forms we have among Sea-urchins. There 

 are some in which large plates alternate with very 

 small ones (Plate XIII fig. A) which are called Ci- 

 daris,, There are others, in which the plates are 

 more numerous, in which the rows of holes are 

 broader (Fig. B.) and in which the spines are small, 

 Echinus. There are others in which we have plates 

 still more numerous, (Fig. C ) the body more coni- 

 cal, the rows of holes being still larger, and the 

 spines reduced almost to little heads, Holopneustes. 

 On the shores of the Northern Sea,where the above 

 described larvae of Sea-Urchins were observed, 

 there is no Echinoderm found belonging to the ge- 

 nus Cidaris. Nevertheless, you will notice that 

 that young Sea-urchin of Plate XI fig. G. has re- 

 markably large spines, equalling nearly the whole 

 diameter of the animal, although in its perfect con- 

 Tlition it will have proportionally small ones. From 

 that very fact we can conclude that the Cidaris 

 stands lower than the Echinus ; though it is usu- 

 ally considered a more elegant and higher form. 

 This conclusion must be granted at once, when we 

 consider the great disproportion in the size of the 

 spines in Cidaris, and the large plates for the spines 

 resembling the embryonic form of the Echinus, 

 that the genus Cidaris ranks lower than Echinus. 



In Holopneustes, (Plate XIII, fie. C,) in which 

 the rows of holes are wider still than in Echinus, 



