EVOLUTION 923 



insectivorous creature, about six inches long, often found under 

 stones in South Africa. There are two minute spurs, each supported 

 by a bone corresponding to the thigh or femur ; but these spurs are 

 buried in the tissues and never project. Therefore we may assume 

 that they are not of any use. 



(/) On the back of the skate's flattened head, just behind the eye, 

 there is a large opening or spiracle, by which the water used in 

 respiration passes into the pharynx, to be driven out by the five 

 ventral gill-clefts. This spiracle is the first gill-cleft (hyo-mandibular), 

 turned dorsally instead of ventrally, and used for the intake, not for 

 the outflow, of water. It is not in the slightest degree vestigial, and 

 as we have noticed elsewhere it has a particular interest in being the 

 homologue of the Eustachian tube of Amphibians, Reptiles, Birds, 

 and Mammals. But if we look into the spiracle of the skate we see 

 on the anterior wall a comb-like structure with parallel ridges. 

 There is no doubt that this is the residue of the first gill, the ridges 

 corresponding to the gill-lamellae, but the whole structure is so small 

 and slight that all function has gone. It is a typical vestigial organ. 



(g) The familiar shell of a snail is represented in the Cellar Slug 

 (Limax) by a minute plate of lime hidden on the animal's back, and 

 in the Carnivorous Slug (Testacella) by a sprinkling of calcareous 

 granules, while in the Black Slug (Arion) there is no vestige at all. 

 Similarly, the beautiful chambered shell of the Pearly Nautilus, in 

 part of which the animal lives, is represented in Spirula by a not 

 less beautiful, yet minute and internal, chambered spiral. The 

 animal is not in it, but it is in the animal. Perhaps it may serve as 

 a strengthening internal support, like the sword-like vestiges in 

 sepia and squid; yet it must be noted that the Octopus type is 

 highly successful, although no trace of a shell is left. It may be 

 pointed out that there is in almost all molluscs an embryonic shell-sac 

 with a microscopic shell, but this does not correspond to the subse- 

 quent mantle-made shell that we have been discussing here. 



Let us take another remarkable example, the Watering-pot bivalve, 

 Aspergillum, which has become adapted to a sedentary life in a 

 burrow. The tubular shell is the length of one's middle finger, with 

 a diameter like that of a fountain-pen, and the upper end is closed 

 with a disc perforated like the rose of a watering-can. Near this end, 

 if we look carefully, we see plastered on and immovable a pair of 

 minute valves, the original valves of the bivalve — obviously quite 

 functionless. This is a peculiar case, where the exaggerated growth 

 of a secondary shell has incorporated the original one and made it 

 useless ; so this now finds its place among vestigial structures. 



{h) Among Arthropods there seem to have been many instances 

 of appendages becoming small, and then being transformed so that 

 they became suited for a new function; but there are other cases 

 where the dwindled part has no known use. The first pair of abdom- 



