150 PRIMITIVE FORMS OF LIFE 



phases of a transformation whose point of departure is 

 manifestly the position imposed upon the animal by the 

 necessities of its search for food. The deep-water Holothurians 

 have nothing to get from the clear water around them, in which 

 none of the microscopic Alga sufficing for the nourishment of 

 Psolus are to be found. They feed on mud, and for this purpose 

 the tentacles that surround their mouths are reduced to simple 

 tubes spread out into the form of a button ; the contrast 

 between the dorsal and ventral surfaces is accentuated ; 1 the 

 suprabuccal bend of the Peniagone spreads out into a sort of 

 banneret, with the anterior border elegantly pinked ; the 

 unused dorsal tube-feet elongate into purely ornamental 

 cones in the case of the Deimatinae ; they are atrophied in 

 Psychropotes, and the body terminates in a broad pointed and 

 hollow tail ; the lateral tube-feet of the ventral surface 

 sufficing for locomotion, the median ones may disappear 

 altogether through disuse. 



Even in mean depths of four hundred to two thousand metres, 

 where the light has ceased to penetrate, forms representative 

 of the fauna of Secondary times are scarce, and none are 

 related to those which characterized the Primary Epoch. 

 From this we must conclude that the fauna of the deep sea 

 is relatively recent, and since we have not discovered in the 

 depths those archaic forms with which Agassiz credited it, not 

 even a single one that might be considered the head of a series, 

 but only much modified organisms adapted to a special type 

 of life, we are forced to conclude that these forms have come 

 down from the shores, and as they descended into the deep 

 water, have gradually taken on special characters in harmony 

 with their mode of life. These adaptations are especially 

 remarkable in the Decapod Crustaceans. They are divided 

 into two groups : the swimming Decapods, of which Shrimps 

 are the common type, and those Decapods that walk on the 

 ground, represented by such familiar forms as the Lobster, the 

 Crayfish, the innumerable legion of Galatheidse, and the familiar 

 Crab. The first make but little use of the long, thin legs with 

 which their thorax is provided ; they swim either by means of 

 large flattened appendages which replace the legs on the segments 

 of the abdomen, commonly supposed to be the tail, or by sudden 



1 Psychropotes, Oneirophanta, Deima, Peniagone, etc. 



