428 



HISTORICAL GEOLOGY. 



2. Echinoids (Sea-urchitis) . — Having a thin and firm hollow shell, covered externally 

 with spines (Fig. 441) ; form, spheroidal to disk-shape ; the mouth below, at or near the 

 center, as the Echinus. Fig. 441 represents an Echinus partly uncovered of its spines, 

 showing the shell beneath, and 432 another, wholly uncovered. The shell consists of polyg- 

 onal pieces, in 20 vertical series, arranged in 10 pairs, except in species of the Paleozoic. 

 Five of these 10 pairs are perforated with minute holes, and are called the amhulacral 

 series (a in Fig. 441 represents one pair) ; and the other 5, alternating with these, are 

 called the inter-ambulacral (6). The inter-ambulacral areas have the surface covered 

 with tubercles, and the tubercles bear the spines, all which are movable by means of 

 muscles. The ambulacral have few smaller tubercles and spines, or none ; but over each 

 pore (or rather each pair of pores) the animal extends out a slender fleshy tentacle or 

 feeler, which has usually a sucker-like termination and is used for clinging or for loco- 

 motion. In Fig. 432, the inter-ambulacral areas are broad and the plates large, but the 

 ambulacral are narrow and the plates indistinct. The liiowi/i-opening is situated below, 

 at the center of radiation of the plates. The anal opening in the Begular Echinoids 

 (Fig. 441) is in the opposite or dorsal area or center of radiation. Around the dorsal area 

 there are 5 minute genital openings. In the Irregular Echinoids — constituting a large 

 group — the anal opening is to one side of this dorsal center of radiation, and often on the 

 ventral or under surface of the animal. In Fig. 432, for example, the anal opening is 

 marginal instead of central, while the genital pores are around the dorsal center, as in 

 the Begular Echinoids. To one side of the dorsal center in the Regular Echinoids, there 



432-434. 



434 





m 



lD%)g(7J 



o^d^u 



M 





ECHINODERMS. — Fig. 432, an Echinus without its spines, — the Clypeus Hugi of the Oolyte; 433, the living 

 Pentacrinus caput-medusae of the West Indies (x i) ; «, 6, c, d, outlines of the stems of different species 

 of Pentacrini; 434, plates composing the body of the Crinoid, Batocrinus longiiosti-is. 



is a small porous prominence on the shell, often called the madreporic body, from a degree 

 of resemblance in structure to coral. In some of the Irregular Echinoids, this madreporic 

 body is in the center of dorsal radiation. 



The ambulacral areas are sometimes equally perforated throughout their length. 

 But in other cases only a dorsal portion is conspicuously perforated, as in Fig. 432, and, as 

 this portion has in this case some resemblance to the petals of a flower, the ambulacra are 

 then said to be petaloid. A large part of Echinoids have a circle of 5 strong, calcareous 

 jaws in the mouth ; in a portion of the Irregular Echinoids there are no jaws. 



3. Asterioids ( Star-fishes) . — Having the exterior stiffened with articulated calcareous 

 granules or pieces, but still flexible ; form star-shaped or polygonal ; the viscera extending 



