COMPARATIVE ANATOMY. 



217 



There is, besides thin ambulacra! or water-vascular system, a 

 blood syutem, with both heart and vessels. Also, the liquid con- 

 tained in the box external to th food canal ia supposed to be 

 organised. These points of structure need further study. 



It will be seen that almost all the parts of the echinus are 

 radially disposed, yet the individuals are separate and locomotive, 

 re, the radial structure, which ia best suited to 

 a fixed condition, and a vegetative habit, united with habits 

 such as characterise the higher animals, for the echinus does 

 not float or move Imp-hazard, as the free-swimming hydrozoon 



the stone-lilies. These animals were very numerous in geologu 

 times, and the hard joint* of their long stalks afforded no small 

 puzzle to geologist*. The problem was solved by the discovery 

 both of the whole fossil hard parts of the animal united, and 

 also of some existing representative! of the order in tropical 

 regions. The Crinoids, as they are called, grow like plants in 

 the seas of the tropics. A stein of gelatinous matter encloses the 

 closely-applied hard joints, and bean on its summit a cup, 

 walled in by more hard pieces, around whose edges long arms 

 are developed. Their shape is too complicated for description 



I. DIAGRAM SHOWING THE PLATES AND HOLES ON THE UPPER SIDE OF AN ECHINUS SHELL. II. AMBULACRAL PLATES ENLARGED. III. EcHnrua 



DIVIDED IN THE EQUATORIAL REGION TO SHOW ALIMENTARY CANAL. IV. SPINE, WITH SECTION OF ITS TUBERCLE. V. JAWS AND TEETH 

 WHICH, UNITED, ARE CALLED THE " LANTERN OF ARISTOTLE." VI. SlDE VlEW OF A SINGLE JAW. VII. ITS TOOTH. VIII. ISSIDE OF 



THE PURPLE-TIPPED SEA-URCHIN, SHOWING THE CALCAREOUS LOOPS (a). 



Befs. to Nos. in Figs. (I.) 1, anal hole ; 2, madreporic plate ; 3, genital plates with their pores ; 4, ocular plates and pores ; 5, ambulacra! tract* 

 and holes ; 6, iuterambulacral or imperforate plates. (III.) 1, base of jaws ; 2, gullet; 3, commencement of stomach ; 4, anus. (IV.) 1. 

 pit ligament ; 2, annular muscle. 



does, but evidently searches for food, and has a definite object 

 in locomotion. We might, therefore, expect that in this class 

 we should find different grades, leading from a fixed condition, 

 with its radial symmetry, up to the more perfect method of loco- 

 motion which accompanies an elongated form and a two-sided 

 arrangement. 



We might expect that a radiated animal was a fixed, flower- 

 like animal fallen off from its stalk. This we found to be the 

 case with the Medusae, and we could trace the transformation in 

 the life-history of the animal. In the class Echinodermata, we 

 may also find it is so, only we have to look not simply to the 

 life-history of one animal, but to trace up the development of 

 the different groups throughout the class. The connecting links 

 between, the Coelenterata and the Echinodermata are found in 



here, but it suffices to say that the cup corresponds to the box 

 of the echinus. It is satisfactory to find that an animal, found 

 in our seas, and long considered to be a free brittle star, com- 

 mences life like a stone-lily, and absolutely falls off its stem at 

 a certain stage to commence a new locomotive life. 



The star-fish represents the next higher grade, and although 

 its general form is so different from that of the echinus, it is not 

 difficult to show how the one may be derived from the other. If 

 we suppose the echinus to be quartered, as we quarter an orange, 

 by dividing it along the zigzag lines between the larger plates, 

 and then each division opened, bent down, and flattened out. 

 while the intermediate membrane is supposed to be indefinitely 

 elastic, so as to stretch and cover in the upper part of the 

 animal, we should have a star-fish. All the ambulacra would be 



