THE WONDERS OF LIFE 



heart; in the articulates there are several pairs, repeated 

 in the different segments of the body. Gastric or in- 

 ternal gills are peculiar to the vertebrates and the next- 

 related tunicates, with a small group of the vermalia, 

 the enteropneusta. In these the fore-gut or head-gut 

 is converted into a gill-organ, the wall of which is pierced 

 with gill-fissures; the water that has been taken in by 

 the mouth passes away through the outer openings of 

 these fissures. In the lower aquatic vertebrates (acrania, 

 cyclostoma, and fishes) the gills are the sole organs of 

 breathing; in the higher animals, that live in the air, 

 they fall into disuse, and their place is taken by lungs. 

 Nevertheless, heredity is so tenacious that we find from 

 three to five pairs of rudimentary gill-clefts in the em- 

 bryo right up to man, though they have long since 

 ceased to have any function. This is one of the most 

 interesting of the palingenetic facts that prove the de- 

 scent of the amniotes (including man) from the fishes. 



The group of the aquatic echinoderms has some very 

 peculiar features of respiration. Their body possesses 

 an extensive water-duct, which takes in the sea-water 

 and returns it by special openings (skin-pores or madre- 

 porites). The many branches of these water-vessels or 

 ambulacral vessels fill with water, especially the tiny 

 feelers or feet, which extend from the skin in thousands; 

 they serve at once for movement, feeling, and breathing. 

 But many of the echinoderms have also special gills — 

 the star-fish have small finger-shaped cutaneous gills on 

 the back, the sea-urchins special leaf -shaped ambulacral 

 gills, the sea-cucumbers internal gastric gills (tree-shaped 

 branching internal folds of the rectum). 



The organs of air-breathing are called, in general, 

 lungs (pulmones). Like the organs of water-breathing, 

 they are formed sometimes from the external and some- 

 times from the internal covering of the body. Cutaneous 

 or external lungs are found in several groups of the 



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