150 CCELENTERATA. 



internal. True sexnal reproduction occurs in all members, but 

 in many forms colonies are produced by a process of continuous 

 budding or fission, the zooids remaining connected. The life of 

 the individual is combined with the life of the whole, and the 

 nutriment prepared by each organism is made to contribute to 

 the nourishment of the community of which it forms a part. 

 Such composite organisms, of very soft tissue, require a sustain- 

 ing skeleton, hence an endodermic skeleton of carbonate of calcium 

 is commonly secreted in the body walls and mesenteries, which 

 copies the structure of the polyp ; this is the true " coral " 

 (corallum). 



Four orders are recognized, Zoantharia, Rugosa, Alcyonaria 

 and Ctenophora. The latter are active swimmers; and of the 

 first ordei" the Anemones have a limited power of locomotion. 

 The Alcyonaria include the precious corals. Corals abounded in 

 the Lower Silurian, but were most abiindant, probably, in the 

 Devonian. 



ORDER ZOANTHARIA. 



In this oi'dei' the tentacles are usually simple, smooth and 

 numerous, and like the mesenteries, in multiples of six. The 

 Zoantharia malacodermata include those without a skeleton, as 

 the sea-anemones {Actinidce). Zoantharia sclerodermata ai-e those 

 which secrete a corallum within the body, and include the com- 

 mon reef-forming corals. Zoantharia sclerobasica have the coral- 

 lum as a central axis or base supporting the investing polyps. 

 The latter appear in the Tertiary. The second group has been 

 abundant since Lower Silurian time. 



No. 466. Astrocermm pyriformis. Hall. 



Niagara Group, Waldron, Indiana. 



No. 467. Chaetetes lycoperdon, Say. 



According to Meek, and others, this is Pander's species petropoUtanus, and 

 many of our best authorities place it under d'Orbigny's genus MonticuUpoi'a. 

 Trenton Group, Decorah, Iowa. 



No. 468. Dendropora ornata, Rom. 

 Corniferous, Clarke Co., Indiana. 



