io6 



NATURAL SCIENCE. 



February, 



about, for protection, the hardening of the integument, while 

 locomotion gave rise to annulation. In this condition of the body, 

 the animal could subsist in a damp, slimy medium. It is justifiable to 

 assume that limbs were developed in adaptation to life on dry land, 

 where the skin would suffer from friction and the respiratory organs 

 from exposure to dust. Simple lateral sac-tracheae (Fig. 2 Aa) carried 

 on respiration also during the rainy season : more highly developed 

 tracheae (lungs) with folded walls (Fig. 2 Ab), especially when muscles 

 were associated with them, afforded the animal the means of closing 

 the sacs, concomitantly with the development of the first rudiments of 

 the locomotory organs by the evagination of the folds (Fig. 2 B). 



Ontogeny teaches us that the limbs, as a rule, do not begin to 

 form simultaneously along the trunk, but develop from before 

 backward. The anterior portion of the body thus became adapted for 

 life on land sooner than the posterior portion : it is therefore not 

 surprising that, as stronger subsequently developed musculature 



Fig. 2. — Comparative Diagrams illustrating Evolution of Lungs and Limbs. 



A— Combined diagram, a, simple sac-trachea (lung); ft, sac-trachea provided 

 •with folds. 



B — Combined diagram, a, a folded sac-trachea ; b, the remains of the so-called 

 lung ; e, epipodite ; ex, exopodite ; en, endopodite. 



C — Combined diagram, ep, epipodite ; eX; exopodite ; en, endopodite. (In 

 Trochosa singoriensis, Laxm.) 



appeared at the anterior part of the body, the respiratory organs 

 vanished, but left behind the limbs as vestiges, while, in the posterior 

 part, the respiratory organs persisted in a high state of development. 

 Whereas, if the whole body had become simultaneously adapted to 

 land life, or else one part shortly after the other, it would have 

 retained the original worm-type (Myriopoda). 



But how have the Crustacea arisen ? Their structure, considered 

 anatomically and ontogenetically, leaves no doubt of their racial 

 relationship to the other Arthropoda. The annulation of the 

 exoskeleton and the development of the limbs must, in them, have 

 been determined by the same causes as in the other Arthropoda. 

 The Crustacea, as Simroth has so conclusively argued in his 

 •' Entstehung der Landthiere," 1891, were originally terrestrial, their 

 limbs serving for locomotion on land. They may have been compelled 

 by the search for food, to take to an aquatic life. The exo-, endo-, 

 and epi-podite developed from lung lamellae for increase of the 



