156 PRIMITIVE FORMS OF LIFE 



Echinoderms are entirely absent. The Molluscs deserve special 

 attention ; the Cephalopods are completely lacking, but many 

 successive migrations of marine Gastropods can be counted. 

 The Diotocardiacs are represented by the Neritinidae, the 

 herbivorous Monotocardiacs by forms like Ampulla, Paludina, 

 and Valvata. No carnivorous Monotocardiac, Opisthobranch, or 

 Pteropod occurs in fresh water. On the other hand, the 

 Pulmonata are fairly numerous, and we might well ask 

 whether they are not descended from terrestrial Pulmonata. 

 This fauna is thus very limited. The Lamellibranchs are 

 represented only by forms with a large open mantle, 

 therefore primitive forms, Unio, Anodonta, Cyclas, Iridina, 

 etc. ; and by a single siphoned form which merely 

 indicates the order, Dreyssensia polymorpha which pene- 

 trates into the rivers and carries with it Cordyhphora 

 lacustris and an annelid worm remarkable for its bristles of 

 complicated form, Psammoryctes imibellifer. This invasion 

 seems to have begun only since the beginning of the century, 

 starting in the Baltic, then reaching the Thames and finally 

 the Seine. 



The group of freshwater Fish is among the most instructive. 

 The primitive Fish, fleeing from the struggle for existence 

 that was too intense along the coasts, sought refuge at some 

 distant period in the lakes and rivers, just as the sturgeons, 

 salmon, and shad still take refuge to spawn and place their 

 progeny in some place of shelter. The number of marine 

 creatures able to live in water containing no sea-salt is actually 

 quite small ; those which possessed this pre-adaptation to life 

 in fresh water, or have acquired it, could not be pursued by 

 those not possessing it, and this is why the rivers and marshes 

 which were at first deserted, were early invaded by fugitives 

 which preferred the calm of the inland solitudes to the dangers 

 lying in wait for them among the active and numerous 

 population of the coasts. Thus, the same desire for security 

 peopled the open sea, the abysses and the fresh waters. 

 If Amphioxus, the most primitive of Vertebrates, found a 

 hiding-place on sandy shores, the Lampreys, like Petromyzon 

 marinus, became the temporary guests of the fresh waters, 

 where they only penetrated to spawn, their young having to 

 pass the first part of their life in the fluviatile sand in the form 

 of Ammoccetes. Others like the Petromyzon fluviatilis are 



