128 



ANIMAL KINGDOM. 



Fig. 143 represents an Echinus partly uncovered of its spines, showing the shell be- 

 neath, and 147 another, wholly uncovered. The shell consists of polygonal pieces, 

 in twenty vertical series, arranged in ten pairs, except in species of the Paleozoic. 

 Five of these ten pairs are perforated with minute holes, and are called the am- 

 bulacral series (a in Fig. 143 represents one pair); and the other five, alternating with 

 these, are called the inter-ambulacral (b). The inter-ambulacral areas have the sur- 

 face covered with tubercles, and the tubercles bear the spines, which are all 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 locomotion. In Fig. 147, the inter-ambulacral areas are broad and the 

 plates large, but the ambulacral are narrow and the plates indistinct. 



The mow^-opening is situated below, at the centre of radiation of the plates. 



The anal opening in the Regular Echinoids (Fig. 143) is in the opposite or dorsal 

 area or centre of radiation. Around the dorsal area there are five minute ovarian open- 

 ings. 



In the Irregular Echinoids — constituting a large group — the anal opening is to 

 one side of this dorsal centre of radiation, and often on the ventral or under surface of 

 the animal. In Fig. 147, for example, the anal opening is marginal instead of central, 

 while the ovarian pores are around the dorsal centre, as in the Regular Echinoids. 



To one side of the dorsal centre in the Regular Echinoids, there is a small porous 

 prominence on the shell, often called the madreporic body, from a degree of resem- 

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

 is in the centre of dorsal radiation. 



The ambulacral areas are sometimes perforated throughout their whole length. But 

 in other cases only a dorsal portion is conspicuously perforated, as in Fig. 147, 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 five strong, 

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



3. Asterioids or the Star-fishes. — Having the exterior stiffened with 

 articulated calcareous granules or pieces, but still flexible ; form star- 

 shaped or polygonal ; the viscera extending into the arms ; mouth be- 

 low, at centre ; arms or rays with a groove on the lower side, along 

 which the locomotive suckers protrude through perforated plates ; eyes 

 at the tips of the arms, Ex., the Star-fish, Fig. 144. 



4. Ophiuroids or Serpent- Stars. — Having a disk-like body with a 

 star-shaped mouth beneath, and long, jointed, flexible arms, which 

 sometimes subdivide by forking, but never bear pinnae, and have no 

 grooves along the under side, nor eyes at the slender tips. The 

 viscera do not extend into the arms ; the ovarial openings are slit-like, 

 between the bases of the arms ; and there is no anal orifice. 



5. Crinoids (including Comatulids). — Like ordinary star-fishes in 

 having flexible arms or rays; but the calcareous secretions of the 

 rays and body constitute a series of closely-fitting solid pieces, and 

 the viscera are confined to the body portion. The rays are often 

 very much subdivided, and bear pinnae, in which the generative organs 

 sire situated. 



There are three tribes of Crinoids: — 



(1.) The Crinidea or Encrinites. — Having a regular radiate struc- 



