158 ANIMAL KINGDOM. 



symmetrical inform, and equal either side of a vertical line a b. The valves, 

 moreover, are almost always unequal ; the larger is the ventral, and the 

 other the dorsal. There is often an aperture at the beak (near b, 

 fig. 160) which gives exit to a pedicel by means of which the animal 

 fixes itself to some support. In fig. 161, representing a species of 

 the genus Lingula, the fleshy support is a long one, and the shell 

 stands like a plant, with the opening upward. 



These Brachiopods are also peculiar in other points of structure. 

 They have a pallium, but no independent branchial leaflets. They 

 have a pair of coiled and fringed arms, which they sometimes ex- 

 trude (fig. 216), — whence the name Brachiopod, meaning arm-like 

 foot. For the support of these arms there are often bony processes 

 in the interior of the shell, of diverse forms in different genera 

 (figs. 208, 212, and 215). 



2. Ascidians, or Tunicates. — These Mollusks are enclosed in a 

 leathery skin instead of a shell. They do not occur fossil. 



3. Bryozoans. — Bryozoans, or moss-animals (so named from the 

 moss-like corals they often form), look like polyps, as represented 

 in figs. 162, 162 a. 162 is magnified about eight times. The corals 

 consist of minute cells, either in branched, reticulated, or incrust- 

 ing forms, and are common in the Silurian as well as later rocks 

 and in existing seas. Fig. 162 a represents the animal, showing its 

 stomach at s, and the flexure in the alimentary canal, with its ter- 

 mination alongside of the mouth. Eschara, Flustra, Retepora, are 

 names of some of the genera. 



IV. Radiates. 



The sub-kingdom of Radiates contains three classes : — 



1. Echinoderms. — Having the exterior more or less calcareous, 

 and often furnished with spines and distinct nervous and respira- 

 tory systems and intestine, as the Echinus (fig. 153), Star-fish (fig. 

 154), Crinoid (fig. 155). The name is from echinus, a hedgehog, in allu- 

 sion to the spines. 



2. Acalephs. — Having the body usually nearly transparent or 

 translucent, looking jelly-like. Internally a stomach-cavity, with 

 radiating branches ; also a circular nervous cord. Ex., the Me- 

 dusa, or Jelly-fish (fig. 150). They generally float free, with the 

 mouth downward. 



3. Polyps. — Fleshy animals, like a flower in form, having above, 

 as seen in figs. 147, 148, a disk with a mouth at centre and a margin 

 of tentacles ; internally, a radiated arrangement of fleshy plates ; 

 and living for the most part attached by the base to some support. 

 Ex., the Actinia, or Sea-Anemone, and the Coral animals. 



