16 ECHINODERMATA. 



The class Echinodermata is divided into eight orders, which, in descending sequence, 

 may be thus arranged: 



1. SlPUNCULOIDEA. 



2. HoLOTHUROIDEA. 



3. ECHINOIDEA. 



4. ASTEROIDEA. 



5. Ophiuroidea. 



6. Blastoidea. 



7. Cystoidea. 



8. Crikoidea. 



Order i. Sipunculoidea — form the apodal Annulose Echinoderms; they have a 

 long cylindrical body, divided into rings by transverse folds of the integument ; they have 

 neither tubular suckers nor calcareous parts developed in their body, nor is it divided 

 into a quinary arrangement of longitudinal lobes ; some have homy booklets like the 

 feet of many Annulosa, which they much resemble ; their mouth is provided with a 

 retractile proboscis, and surrounded by small tentacula, differing both in structure and 

 arrangement to the homologous parts in the Holothuria. In them the type of Madiata 

 vanishes and that of Amudosa appears. They are unknown in a fossil state. 



Type. Sipunculus edulis, Pallas. 



Order u. Holothuroidea. — Body in general elongated ; skin in general soft and 

 leathery, in a few genera strengthened by calcareous or horny spines. Eive avenues of 

 suckers divide the body into as many nearly equal segments ; mouth surrounded by 

 plumose tentacula, the numbers of which are usually multiples of five ; vent at the 

 opposite extremity of the body ; digestive organs consist of a large intestine, which makes 

 several coils in passing through the body ; respiration performed by internal ramified 

 tubes, like a miniature tree ; locomotion effected by contractions and extensions of the 

 body, and by rows of tubular suckers, similar to those in the Star-fishes and Sea-urchins. 

 The softness of their naked integument prevents their preservation in the stratified rocks. 

 Type. Cucmnaria frondosa, Gunner. 



Order in. Ectiinoidea. — Body spheroidal, oval, or depressed, enclosed in a test, 

 composed of twenty columns of calcareous plates, with ten rows of holes for the passage of 

 retractile tubular suckers; the surface of the test is studded with tubercles, which have 

 jointed with them moveable spines, of various sizes and forms in the different families 

 and genera; at the summit of the test is the apical disc, composed of give genital plates, 

 perforated for the passage of the ovarial and seminal tubes, and five ocular plates for 

 lodging the five eyes. The mouth, situated always at the under surface, is in many 

 genera armed with five powerful, complicated jaws and teeth, and in others the peris- 

 tome is edentulous ; the vent occupies various different positions, sometimes within the 

 apical disc and surrounded by its elementary parts, sometimes external to the disc, and 

 at the upper surface, side, or ba«e, the relative position of the vent to the disc affording 



