HEART-URCHINS. 



193 



perforated portions of the ambulacral areas are now 

 restricted merely to the central region of the ujjper 

 surface, one of these areas (directed tij^wards in the figure) 

 forming' a shallower groove, and lieing otherwise markedly 

 different fi'om all the others. In such an urchin the 

 bilateral symmetry is very strongly marked indeed, and, 

 since the upper border of the figure represents the 

 anterior, and the lower the posterior extremity of the 

 animal, we may compare the three anterior ambulacral 

 areas to the head and arms of a quadrujied whose hind 

 legs will be represented by the two posterior ambulacra. 

 In regard to the position of the two orifices of the test in 

 this species, the vent is situated on the flattened jJosterior 

 surface, near its junction with the upj^er surface, while the 

 mouth occupies a position nearly midway between the 

 centre and the anterior border of the lower surface, at the 

 commencement of the groove formed by the anterior 

 ambulacral area. The mouth is peculiar in that it does 

 not open directly on the surface, in the usual manner, but 

 has a projecting lip by means of which its aperture 

 assumes a forward direction. The conunon jjurple heart- 

 urchin (SjHitaiigus) of our present seas is a larger 

 representative of this group, presenting the same general 

 type of structure. 



We thus see how gradual is the passage from a species 

 of the type of the common lu'chin to that of the heart- 

 urchin, widely different as are these two from one another. 

 We might, indeed, proceed further with our investigations, 

 and show how certain of the irregular urchins have Vjeconie 

 so flattened as to assume the form of thin plates, which 

 in some instances are deeply notched at their periphery. 

 And we might also investigate the variation of form and 

 size displayed by the sjjines of the different groups. 

 Enough has, however, been written for our present object, 

 which has been to show the amount of interest that 

 attaches to the investigation of the lines of modification 

 on which develojjment has proceeded among the sea-urchins. 

 This has shown how a regular progressive advance in one 

 jwH'ticular direction has taken p)lace from the earlier to the 

 later members of the group ; and we thus have another 



