I50 MANUAL OF ZOOLOGY. 



are only four genital glands, and, therefore, only four genital 

 plates in the apical disc. 



T]\Q EMnoidea may be divided into the following principal 

 families : — 



Synopsis of the Families of Echinoidea (after Pomel). 



Sub-order I. Echinida. —Test composed of no more than twenty rows 

 of plates. 



a. Spatifornies. — Mouth excentric, in front ; anus behind ; anterior am- 



bulacrum obliterated ; form obovate. (The so-called " Spantan- 

 goid" Sea-urchins.) 



1. Ananchytida. — With simple ambulacra. 



2. Spantangida. — With petaloid ambulacra. 



b. Lampadiformes. — Mouth central, or nearly so; toothed or toothless ; 



anus more or less posterior, but often mounting high enough to 

 enter into the genital disc ; ambulacra similar. 



3. Echinoneida. — Toothless ; ambulacra simple. 



4. Cassidulida. — Toothless ; ambulacra petaloid. 



5. Clypeastrida. — Toothed ; ambulacra petaloid. 



6. Echinoconida. — Toothed ; ambulacra simple. 



c. Globiformes. — Mouth central ; anus opposite the mouth, surrounded 



by the genital plates. 



7. Cidartda. — Ambulacra prolonged on the buccal membrane and 



destitute of buccal branchisE. 



8. Echinida. — Ambulacra not prolonged on the buccal membrane, 



but provided with buccal branchiae. 

 Sub-order II. PERiscHOECHiNiDiE or Tesselata. — Corona of the test, 

 consisting of more than twenty rows of plates. (The PalEeozoic Sea- 

 urchins, Archaocidaris and Palachinus.) 



CHAPTER XX. 



ASTEROIDEA AND OPHIUROIDEA. 



Order Asteroidea {Stelleridd). — This order comprises the 

 ordinary star-fishes, and is defined by the following characters : 

 — The body (fig. 42) is star-shaped or pentagonal, and consists 

 of a central body or " disc," surrounded by five or more lobes, 

 or " arms," which radiate from the body, are holl.ow, and con- 

 tain prolongations of the viscera. The body is not enclosed 

 in an immovable box, as in the Echinoidea, but the integument 

 (" perisome ") is coriaceous, and is strengthened by irregular 

 calcareous plates, or studded by calcareous spines. No dental 

 apparatus is present. The mouth is inferior, and central in 

 position ; the anus either absent or dorsal. The ambulacral 

 tube-feet are protruded from grooves on the under surface of 



