924 LIFE : OUTLINES OF GENERAL BIOLOGY 



inal limbs of a female crayfish are much reduced, very variable, and 

 of no observed function. A hermit-crab clings to the central pillar 

 of its borrowed whelk- shell by means of a terminal pair of abdominal 

 appendages (uropods). In the male there are three other abdominal 

 limbs (pleopods). confined to the left-hand side and very slender. 

 As far as we know they are quite useless in the male. In the female, 

 however, the appendages of abdominal segments II, III, and IV, 

 though confined to one side, are of considerable strength and serve 

 to carry the eggs. This again is an interesting case, that what is 

 apparently useless in the one sex should retain in the other a very 

 important function. 



A pair of transformed abdominal appendages may form an egg- 

 laying organ or ovipositor in a female insect, and this may be 

 re-transformed into a sting, naturally unrepresented in the male; 

 but there is no question of an ovipositor or a sting being vestigial. 

 On the other hand, in some other insects, there are reduced paired 

 appendages at the end of the abdomen for which no use is known. 

 They are probably true vestigial structures. Similarly with the three 

 pairs of mouth-parts in insects, which, however transformed, are 

 always made up of mandibles, first maxillae, and labium, there are 

 frequent reductions of parts to what may be almost literally called 

 "vanishing-points". Thus moths and butterflies have vestigial 

 mandibles, not known to be of any use in the adult. 



Vestigial Structures in Plants. — In some flowers that have 

 lost their radiate symmetry and become bilateral, as in Scrophularias, 

 one of the five stamens has become a useless vestige; and there are 

 many similar instances. Sepals and petals may sink below the limit 

 of useful size, and a leaf may be reduced to a mere scale. One must 

 be careful, however, to note that great reduction of size does not 

 necessarily imply that the structure is vestigial; for the concept 

 "vestigial" implies uselessness. 



TRANSIENT BUT USEFUL EMBRYONIC STRUCTURES.— 



From true vestigial structures, which we are trying to discriminate 

 more precisely, we must distinguish reduced embryonic structures 

 which still have their use during development, but do not come to 

 anything in adult life, {a) The primitive dorsal axis known as the 

 notochord arises in development as an axial differentiation of endo- 

 derm cells along the dorsal middle line of the gut (archenteron) of 

 the Vertebrate embryo. It is the only dorsal supporting axis in 

 primitive Vertebrate animals like lancelets and lampreys, and it is 

 in varying degrees persistent in some fish types, such as the sturgeon 

 and the mud-fishes. In most cases, however, it becomes surrounded 

 by a mesodermic sheath which develops into the backbone — the 

 notochord's substitute. From Bony Fishes to Man the adult sup- 

 porting axis is the mesodermic backbone which replaces the endo- 



