THE MOLLUSCA 35 
The Pulmonata are generally biannual, but Helix pomatia may attain 
an age of six years. The majority of Nudibranchs and Tecti- 
branchs appear to live for one year only. Many Lamellibranchs 
(Mytilus, Teredo) are adult at the end of one year; Avicula is adult 
at the end of two years; Ostraeca edulis is sexual at two years, 
becomes adult in five years, but may live for ten years in oyster- 
beds. The huge Z7idacna 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 Lossia 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 
perish, 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, Velutina, Onchidiopsis, Solariella, 
Machaeroplax, Volutharpa, Torellia, Cyprina, Mya, B. 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 
Nautilus 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 Zvrigonia, 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 Jonoceros, 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 
. 
