COMMON EGG-URCHIN. 151 



themselves, and on the number and arrangement of the 

 pairs of pores in the avenues of suckers. These present 

 good marks of distinction throughout the genus Echinus. 

 The spines are especially important, as from the examin- 

 ation of a single spine it is possible to pronounce on the 

 species to which it belongs. To the geologist this is evi- 

 dently of great consequence, as frequently he meets only 

 with a few scattered spines. But when we leave the fa- 

 mily Echinidse, we leave this important character behind 

 us. Among the Heart-Urchins the spines present one 

 common family structure. A single plate, either ambu- 

 lacral or inter-ambulacral, will also, from the arrangement 

 of the spiniferous tubercles which cover its surface, enable 

 us to pronounce pretty certainly on the animal of which it 

 formed a part. Thus, in this family of Echinodermata, 

 from an apparently insignificant fragment we can construct, 

 as it were, a species, even as the student of the Vertebrata, 

 from a broken bone can pronounce on the form and habits 

 of the animal to which it belonged. 



The apex of the Egg-Urchin presents the anal opening 

 surrounded by five triangular, or somewhat cordate plates, 

 called the ovarian plates. They are so termed since we 

 find in each of them a perforation, which is the opening 

 of the ovaries, and through which the eggs are excluded. 

 One of these plates is somewhat larger than the rest, and 

 much rougher on the surface, being occupied, as it were, 

 by a large porous wart. This body is of the same nature 

 with the madreporiform tubercle of the Starfish. It is the 

 nucleus of the Sea-Urchin, — we might almost say the 

 navel. The ovarian plates are strengthened internally by 

 a strong transverse rib. Their apices are directed from 

 the anus, and they are separated from each other by five 

 smaller cordate plates with apices directed towards the 



