BRIEF REVIEW OF THE SYSTEM OF LIFE. 



427 



422-480. 



Fig. 422, Produotus aculeatus, dorsal view ; 423, Pioductus semireticulatus, ventral view ; 423 a, section of Pro- 

 ductus, showing the curvature of the valves; 424, Chonetes latus, opposite views; 425, Calceola sanda- 

 lina (a Coral with lid, resembling a bivalved Brachiopod) ; 426, Crania antiqua; 427, Discina (Discinisca) 

 lamellosa, side view; 428, id. showing foramen; 429 a, b, Siphonotreta ungulculata, opposite views; 

 430 a, b. Obelus Appollinis. 



Brachiopods are among the oldest of fossils. The animals have been shown by Morse 

 to have close relations to the Annelids, though not multiplicate like them, but when adult 

 without distinct segments. 



2. Bryozoans (Polyzoans). — Bryozoans, or Moss-animals (so named with reference 

 to the moss-like corals they often form), look like Polyps, owing to the series of slen- 

 der ciliated organs surrounding the mouth, as represented in Figs. 

 395, 395 a ; 395 is magnified about 8 times ; and 395 a represents 

 the animal showing its stomach at s, and the flexure in the ali- 

 mentary canal, with its termination alongside of the mouth. The 

 coral consists of minute cells either in branched, reticulated, or 

 incrusting forms. They are often calcareous ; and such were com- 

 mon in the Silurian, and still occur. Eschara, Flustra, Betepora, 

 are names of some of the genera. The Oysters in the market often 

 have their shells encrusted with large groups of the minute cells of 

 Bryozoans. 



Fig. 431 represents a membranous species (called Gemellaria loricata) ; b is the moss 

 like coral, natural size ; and a a portion of a branch, enlarged, showing the cells. 



Bryozoan, Genaellaria 

 loricata. 



5. Echinoderms. 



Echinoderms, while eminently radiate in the adult stage, in the young have bilateral 

 symmetry ; and a few species never get beyond the form of the young. The exterior is 

 more or less calcareous, often furnished with spines. They have distinct nervous and 

 respiratory systems and also a complete digestive system. The name alludes to the spines 

 over the surface in a prominent part of the species, and is from echinus, a hedgehog. 



The following are the subdivisions : — 



1. Holothurioids {Sea-slugs, Sea-cucumbers) . — Having the exterior soft, and through- 

 out extensile or contractile, and the body elongated ; mouth at one end surrounded by a 

 wreath of branched tentacles. 



