ADAPTATION 217 



Adventure, has recently reported a striking variation in a deep-sea 

 prawn. The cephalopods Hving in shallow waters conceal them- 

 selves when molested by discharging a cloud of brown secretion 

 into the water. The prawn behaved likewise, but the darkness of 

 the abysses could hardly be darkened, so the discharge of this 

 animal was a luminous cloud, which in the general absence of 

 light would conceal its movements and often obtain its escape. 



Terrestrial Adaptation : Locomotion. Lull very logically applies 

 the classification of aquatic organisms to those living in the air, 

 so that volant forms may be looked upon as aerial nekton, and 

 ordinary terrestrial forms as aerial benthos. Aerial plankton is 

 very limited. Bacteria are known to float in the air, but they 

 probably remain there only temporarily. The same is true of the 

 spores and pollen of plants. Consequently, while there is an 

 aerial plankton, no organisms can be said to belong to it perma- 

 nently. 



The differences between terrestrial and aquatic life have already 

 been considered under the emergence of the terrestrial vertebrates. 

 A conspicuous feature of the adaptation of terrestrial forms is the 

 modification of supporting and locomotor organs. Since the air 

 by which the animal is surrounded does not support it like the 

 water, its points of contact with the rigid substratum must serve 

 both for support and locomotion. The pentadactyl appendage 

 (Fig. 54) in vertebrates and the jointed appendage in Arthropoda 

 are the outcome of this need while in other forms the body lies on 

 the ground and locomotion is accomplished by creeping, aided some- 

 times, as in the annelids, by setae or other projections to increase 

 the hold of the organism on the surface which supports it. 



While creeping is a very simple process, in general the same in 

 all groups which move about in this way, the development of 

 jointed appendages of either type paves the way for a variety of 

 modifications. Thus we find the vertebrate limb modified for 

 walking, running, jumping, burrowing, and climbing and the in- 

 vertebrate appendage for most of these functions. 



Ambulatory Adaptations. The ambulatory, or walking type 

 obviously involves the least change of form. Such locomotion re- 

 quires in addition to supporting function no further power than 

 successive shifting of position of the several limbs in relation to the 

 ground. Without sufficient development of muscles for this pur- 

 pose, the structures could not function even as supports for the 



