2 THE BIOLOGY OF MARINE ANIMALS 



Phoronidea and Echinodermata. Of the three most highly evolved groups 

 of animals now extant, the molluscs, arthropods and vertebrates, all 

 have marine representatives. The cephalopods are without doubt the most 

 highly developed of marine invertebrates, and approach the vertebrates in 

 the complexity of their sensori-neural organization and behaviour. 

 Arthropods reached their evolutionary peak in the insects, few of which 

 have returned to the sea. But more remarkable, from the historical view- 

 point, has been the repeated re-invasion of the seas by most of the major 

 groups of vertebrates. So successful was this colonization that the teleosts 

 were able to exploit all marine environments from the tidal zone to the 

 ocean abyss, and occupy a dominant position in the oceanic fauna. The 

 maritime birds and mammals, partly through their acquisition of homoio- 

 thermism, have been able to spread through the surface waters of all the 

 oceans from arctic to antarctic ice, and in speed and agility they even out- 

 class the fish and squid in their own element. On the invertebrate animals 

 hitherto occupying the seas the effects of these evolutionary changes have 

 been tremendous. From our point of view they enter into a consideration 

 of the morphological and functional adaptations which have permitted 

 the exploitation of a marine environment. 



MARINE HABITATS 



On the basis of their distribution and habits marine animals are generally 

 classified as plankton, nekton and benthos. The first-named comprises all 

 those small drifting organisms, both plants and animals, which have only 

 feeble powers of locomotion and are carried helplessly at the mercy of 

 currents and tides. Nekton refers to strong swimming animals, such as 

 squid, fish and whales, whose movements are powerful enough to make 

 them independent of water movements to a considerable degree. And 

 thirdly, the benthos embraces all those bottom-living organisms which 

 crawl over the substratum, burrow into it, or are sedentary in habit and 

 remain fixed to one spot, for example, starfish, bivalves and sponges. 



In addition to this classification of marine animals on the basis of 

 habitat it is usual to recognize certain well-defined environments in the 

 sea, each with special characteristics of its own. These are the littoral or 

 inter-tidal region, the continental shelf and slope, the pelagic zone and the 

 abyssal region. Subdivision of these various regions is often necessary for 

 oceanographical and ecological purposes, and is briefly described in the 

 following paragraphs. More extended treatment will be found in works on 

 marine natural history and oceanography, such as Sverdrup et al. (37). 

 Coker (11), Colman (12) and Marshall (31). 



The Inter-tidal Zone 



At the junction between sea and land lies the shore or littoral zone, 

 subject to tidal ebb and flow. This is the region bounded by extreme highl- 

 and low-water levels of spring tides. Its vertical range depends on the 

 extent of the tides, and the area involved is also governed by the slope of 



