244 GEOLOGICAL BIOLOGY. 



Mollusca. — An animal presenting bilateral symmetry'; soft 

 body, non-segmented; possesses neither internal skeleton 

 nor external skeleton ; shows digestive organs verj- well 

 developed, and a nervous aesophageal collar, with three pairs 

 of ganglia in the highest types. Ver>^ many ]\Iollusca secrete 

 in a fold of the skin, called mantle, a calcareous shell with a 

 single or two valves; others are entirely naked, and develop 

 no solid formation. Respiration is mainly effected by gills 

 or branchiae, more rarely by lungs or folds of the skin. A 

 circulator}' system imperfectly closed, with a pulsating organ 

 driving its contents to the peripher}-, exists in Mollusca, 

 except in the lower types. Reproductive organs, differen- 

 tiated into sex organs, sometimes hermaphrodite and some- 

 times separate individuals, and in Bryozoa by budding and 

 formation of colonies, and of various forms. All these animals 

 now called Mollusca were ranked with Worms by Linne. 



The Molluscoidea are particularly characterized by a cal- 

 careous shell, horny integument, or cellulose tissue. Respira- 

 tory organs often in front of mouth, as tentacles or appen- 

 dages; central nervous ganglion, between mouth and anus; 

 besides sexual reproduction, often also budding. All aquatic, 

 and mostly marine. 



Bryozoa. — Small animals, increasing by budding, and united 

 into colonies, branching like moss (hence the name), and form- 

 ing incrustation, etc. Animals enclosed within membranous 

 or calcareous cellules, and possessing at the anterior extremity 

 of the body a mouth surrounded by tentacles; no heart; 

 intestine; well-developed body; anal opening near mouth; 

 hermaphrodite. 



Tunicata. — Sac-like animals, free-swimming or fixed, 

 united into colonies; hermaphrodite; furnished with an enve- 

 lope (mantle) having the consistence of cartilage or leather, 

 which completely surrounds the body, and presents only two 

 openings. Branchiae on the internal part of the cavity 

 formed by the mantle; mouth in front of the branchial sac; 

 heart tubuliform [now ranked 'as a separate branch]. 



Brachiopoda. — Soft animals, living solitary- furnished with 

 a bivalve symmetrical shell, presenting two free lobes of the 



