PRINCIPAL FOBMS OF SEA-UBGHINS. 



133 



Taking a rapid survey of the principal forms of sea- 

 urchins, we may divide the class of EcMnoidea into two or- 

 ders : the Palechinida, or older sea-urchins, in which the 

 -shell is composed of more than twenty rows of plates ; and 

 the Autechinida with twenty rows of plates.* 



Order 1. Palechinida. — Comprises first the suborder Me- 

 lonitida, in which tliere are more than ten rows of ambula- 

 cral plates, represented by Melonites of the coal formation, 

 and ProtecMnus, Palmcliinus, ArcJimocidaris, etc. In the 

 second suborder ^oti^an'a, there are ten rows of ambulacral 

 plates. A type of the group, Eocidaris Eaiserlingii, appears 

 in the Permian formation. 



Order 2. Autechinida. — 

 To tliis division belong sea- 

 urchins with twenty rows of 

 plates. The first suborder is 

 the Desmosticha, comprising 

 those sea-urchins with band- 

 like ambulacra extending 

 from the mouth to the oppo- 

 site extremity, and of more 

 ■or less regular, flattened, 

 spherical form. Such are 

 Cidaris, Echinus, EcMnom- 

 etra, Clypeaster, and Echi- 

 narc'^Jinius. The Echinus 

 ■esculentus Linn., of the Mediterranean Sea, is as large as 

 an infant's head, and is used as an article of food. 



In Clypeaster the body is large and the shell very solid. 

 G. suhdepressiis Agassiz is common on the Floridan coast. 

 An orbicular flattened type are the sand-cakes, of which the 

 Echinarachnius parma Gray (Fig. 86) is abundant in the 

 shallower portions of the North Atlantic, from low-water 

 mark to forty fathoms. It is replaced southward from 

 Nantucket to Brazil by Mellita testudinata Klein. 



The last suborder, Petalosticha, is characterized by the 



Fig. 86.—Echinaracfmius parma, com- 

 mon Saud-cake. Natura) size.— After A. 

 Agassiz. 



* These are terms proposed by Haeckel, who regards these divisions 

 as subclasses, but we think they should more properly be called orders. 



