CIDARIS; ECHINU8; CLYPEASTER. 463 



mouth which leads into a sac-like stomach. At the same time 

 four slender, calcareous rods are formed in the angles of the 

 pyramid, beyond the lower angles of which they project con- 

 siderably, accompanied by a prolongation of the gelatinous flesh 

 of which the whole body is composed. They all meet at the 

 summit, and are also united above the middle by cross pieces, 

 and the entire skeleton has been aptly compared to an art- 

 ist's easel. The little animal still moves by the action of 

 cilia, which occur in the greatest abundance along the course 

 of the calcareous rods. In course of time the young Echinus 

 begins to make its appearance, not however by the metamor- 

 phosis of the larva, but in the form of a small disc in the interior 

 of the dome formed by the flesh of the latter. This increases in 

 size, and gradually acquires a more complex organization ; whilst 

 the curious larval structure, the direct product of the egg, is 

 simultaneously wasted away until it entirely disappears, the only 

 parts of it which enter into the construction of the future S -a- 

 Urchin being the stomach and oesophagus. The discovery of 

 these curious and interesting facts is due to the researches of 

 Professor Miiller of Berlin. 



1 102. The two most important genera of this Order are the 

 Cidaris and the Echinus. The former is usually regarded as 

 the most highly organised of the entire class ; its spines being 

 proportionally few and powerful, and being connected with their 

 tubercles by a mode of attachment superadded to those which 

 we find elsewhere ( 1095). The Echinus, however, is the best- 

 known of these genera, in consequence of its more extensive 

 geographical distribution, and its abundance in the places it 

 frequents. One species, found on the coast of Ireland, has the 

 power of excavating for itself hollows in the limestone rock on 

 which it lies ; but by what means it does so is uncertain. In 

 the Clypeaster and Scutella, the shell is more or less flattened ; 

 and is sometimes divided by vertical partitions, so as in some 

 degree to resemble a Star-fish. At the same time, the vent 

 leaves its central position on the upper surface, and approaches 

 the mouth, which still retains its central position below. In 

 the Spatangus, or Heart-Urchin, common on many of our sandy 



