HOW THE FISHES LEAKNED TO SWIM HEINTZ 



231 



cal tail, that is, the lower lobe of the tail is more strongly developed 

 than the upper. The head and trunk are covered with fine, thin 

 scales. Everything therefore indicates that we have a relatively 

 good swimmer, a highly specialized form. The adaptations of the 

 Anaspids to swimming are nevertheless not as perfect as in the 

 recent free-swimming species. The organs of equilibrium in the 

 Anaspids consist of only a row of spinelike fulcra scales along the 

 back and a relatively strong anal fin. There are, in addition, on 

 both sides of the body immediately behind the gill-opening two pe- 

 culiar immovable spines. They must certainly have corresponded 

 to the paired fins in other fishes, but it is doubtful whether they 

 could have any significant function as organs of steering or equilib- 

 rium as they were much too thin and narrow. The lack of organs 



Ord. Anaspida. 



Figure 4. — Two representatives of the order Anaspida : A, Pterolepis; B, Remigolepis. 

 Cross-section of ttie body and the anal fln in anterior view (after Kiaer). 



of equilibrium is a ver}'^ peculiar phenomenon. We know that all 

 torpedo-shaped free-swimming fishes are in a state of unstable 

 equilibrium in water, as the lower part of the body (with the ab- 

 dominal cavity) is lighter than the solid muscular back. We see 

 this most distinctly in sick or dead fishes which always swim or 

 float with the side or the belly up. In a living condition they main- 

 tain their upright position in the water by motions of the tail, the 

 unpaired and chiefly the paired fins. It must have been difficult for 

 the Anaspids to maintain their equilibrium without considerably 

 developed fins. Perhaps the peculiar hypocercal tail with the down- 

 ward-directed thick body axis helped them in keeping the right 

 position in the water. In spite of all this, however, we must still 

 consider the Anaspids as having been comparatively good swimmers, 

 even if their adaptations to swimming were not as perfect as we find 

 them in the youngest true fishes. 



