PEOPLING OF LAND AND SEA 147 



their beautiful blue colour. Some of the Bryozoa also have 

 learnt to swim. In the Tunicates, tachygenesis has accelerated 

 the metamorphosis which must necessarily have followed 

 fixation, until the egg directly reproduces the permanent 

 form. A strange paradox thus appears. Three new independent 

 types of swimming Tunicates, Pyrosoma, Doliolum, and 

 Salpa, are created, in a sense, by their very immobility and 

 resulting degeneration. The Arthropods are represented 

 notably by small Copepods, which exist in countless shoals, 

 by Schizopods, and Squilla. Contrary to what might be 

 expected, the Annelid Worms, so agile when they squirm among 

 the Algae, furnish the pelagic fauna with few forms, which are 

 generally transparent and of small dimensions : Tomopteris, 

 Ophvyotrocha, Palolo, etc. There are also some strange open 

 sea Nemerteans. On the other hand, Sagitta, with its 

 exceedingly simple organization, entirely isolated among the 

 Worms, abounds at a distance from the coast. The heavy 

 Echinoderms are represented only by some floating 

 Holothurians. The Cephalopod Molluscs are essentially 

 creatures of the open seas, and very varied, but they are also 

 found along the coasts. Among the true Gasteropods on the 

 other hand we can cite only the Ianthinidae, the Atlantidae, 

 Carinariidse, and Pterotrachoididae, which have such special 

 characters that they form a group to themselves, the Heteropoda. 

 The true Gasteropods are replaced by the Pteropods, closely 

 related to the already aberrant Opisthobranchs among the 

 Gasteropods, but removed from them structurally by the 

 possession of swimming organs, which are very mobile and con- 

 sist of two flexible paddles, developed from the ' ' foot ' ' . Like the 

 Copepods and Sagitta, they live together in shoals. All the 

 pelagic Invertebrates have been more or less affected by a 

 curious mimetism. Their bodies, which look as though they 

 were inflated by water, are either transparent or coloured the 

 same shade as the apparently deep blue water of the open sea. 

 The fish naturally form a long series, but certain groups, 

 important from the geological view-point, are lacking. Among 

 these are the Lampreys, in fact the lowest of all ; and 

 absent also are the most primitive Bony Fishes, with pectoral 

 and abdominal fins far apart like those of the Sharks. Sardines, 

 Herrings, and Anchovies really belong to this order, but 

 they live too near the coasts to be considered truly pelagic. 



