THE MOLLUSC A 35 



The Pulmonata are generally biannual, but Helix, porwtia may attain 

 an age of six years. The majority of Nudibranchs and Tecti- 

 branchs appear to live for one year only. Many Lamellibranchs 

 (Mijtilus, Teredo) are adult at the end of one year ; Avicula is adult 

 at the end of two years ; Ostraea edulis is sexual at two years, 

 becomes adult in five years, but may live for ten years in oyster- 

 beds. The huge Tridacna lives for about eight years, the Cyrenidae 

 only two years, but the Anodontidae are remarkable for their 

 longevity ; they do not become sexually mature till they are five 

 years old, and they continue to grow to the age of twenty or thirty 

 years. In the Cephalopoda it seems that llo&sia does not live for 

 more than a year, and Octopus not more than four years. 



Many Molluscs are able to fast for a long time. Cold affects 

 them less than heat. Helix has been known to survive a tempera- 

 ture of - 120 C., and small Gastropods live in thermal springs at a 

 temperature of 42 C. Molluscan embryos and larvae generally 

 pejish, in temperate climates, at temperatures of + 31 C. and 3 C. 



Descriptive zoologists have enumerated more than 28,000 

 species of living Molluscs, of which more than half are Gastropods. 

 Fossil representatives of Molluscs are found in all deposits from the 

 Palaeozoic onwards. 



1. Distribution in Space. 



I. Marine Molluscs A. Littoral Fauna. The shores of con- 

 tinents are divided into three provinces by great thermal variations, 

 a. The North Polar province, with characteristic genera such as 

 Cryptochiton, Molleria, Lacuna, J^elutina, Onchidiopsis, Solariella, 

 Muchaeroplax, Volutharpa, Torettia, Cyprina, Mya. (3. The South 

 Polar-province, with the characteristic genera Photinula, Struthiolaria, 

 Cominella, Eatoniella, Cyamium, Lissarca, Philippiella, Modiolarca. 

 y. The tropical province, or region of coral reefs, in which one may 

 distinguish four sub-regions: (1) The Indo-Pacific, the home of 

 Xdidilus and the chief forms of the Toxiglossa. The Mediterranean 

 should be included in this sub-region, and the Australo-Zealandic 

 division of it, just as is the case with the terrestrial fauna, presents 

 special characters, as shown in Trigonia, Amphibola, etc. (2) The 

 West African and (3) the East American sub-regions, which have 

 several forms in common. (4) The West American, characterised 

 by such genera as Monoceros, Concholepas, etc. Occasionally species 

 may be naturally or artificially acclimatised in various parts of 

 these regions, but exchanges are only definitively effected between 

 similar latitudes, as for example Littorina littorea between Europe 

 and North America, and reciprocally Venus mercenaria and Petricola 

 pholadiformis. 



B. The Abyssal Fauna, is not divisible into distinct provinces, 

 and many of its species are universally distributed either in the 



