MOLLUSCA. 229 



interesting facts. The different groups which compose this 

 class, have had their principal development at very different 

 epochs. The stalked crinoidea are extremely abundant in the 

 palaeozoic rocks: entire strata are in fact sometimes composed 

 of the debris of their skeletons, and their ramose arms covered 

 the bed of many an ancient sea. Their genera and species 

 were greatly diminished in the triasic and oolitic periods. A 

 few species are found in the cretaceous rocks, and they are 

 represented in our modern seas by one rare species. 



The asterida have an inverse history. We know a few 

 species in the trias oolites and chalk, whilst the present seas 

 of all latitudes abound with numerous species. 



The ecJiinida occupy an intermediate position between 

 these extremes. They are rare in the primary epoch in- 

 crease in the trias attain their maximum development in 

 the oolitic period, and gradually decline in the cretaceous 

 and tertiary strata. 



SECOKD DIVISION. 

 MOLLUSCA, Cuvier. CYCLO-GANGKLIATA, Grant. 



Body inarticulate, soft and pulpy, for the most part 

 enclosed in a calcareous envelope or shell. Nervous system 

 composed of ganglia dispersed through the body, and united 

 into a circle by connecting filaments. The oesophagus sur- 

 rounded by a nervous collar ; the large ganglia reposing on 

 this tube represent the brain. The encephalous orders 

 possess organs of sense. Their blood circulates in a system 

 of arteries and veins, aided by the contraction of a two- 

 chambered heart. The terrestrial and most of the lacustrine 

 species breathe by an air-sac or lung ; and all the marine, 

 and most of the lacustrine species, have branchiae for respi- 

 ration. Their shells are composed of a cellular albuminous 

 membrane, indurated with the carbonate of lime, secreted by 

 a portion of the tegumentary system, called the mantle. 

 The shell is for the most part external, and presents a vast 



