258 On the Origin of the Vertebrates. 



section of an Annelid Worm suffices to demonstrate, as was 

 proved by Semper, after Geoffroy Saint-Hilaire, that the in- 

 verse arrangement of the organs in the two branches is easily 

 explained by a simple change of attitude. If we consider the 

 embryogeny of Amphioxus, such as it has been described by 

 Hatschek and, most recently, by Willey, keeping before us the 

 considerations of which a resumi has been given above, all 

 the apparently inexplicable peculiarities in the development 

 of this animal not only become cleared up of themselves, but 

 show the way in which the inversion of the higher Verte- 

 brates has come to pass. The mouth of Amphioxus does not 

 pass indeed all at once to the median line, opposite to the 

 line of the nervous axis; it is formed at the nearest possible 

 point to its primitive situation, on the left side of the body. 

 The mouth having become lateral, the animal cannot feed 

 except on condition that it lies on the left side of the body, 

 which becomes its habitual attitude. According to Lamarck's 

 principle (use and disuse of organs), of which the entire 

 history of the Invertebrates is also a striking confirmation, it 

 results from this new attitude that the organs of sense of the 

 left half of the body, being more in touch with the nutriment 

 supplied by the food, alone develop : the young animal 

 exhibits but one olfactory pit and one gustatory organ 

 (Hatschek's organ), both situated on the left. In consequence 

 of this same attitude all the branchial clefts on the left side 

 are covered up, and it is rendered impossible for them to 

 perform their normal function. The animal is therefore 

 induced to twist the region of its body which corresponds to 

 them, so as to bring all its branchial clefts on to the right 

 side. This is no mere hypothesis ; this torsion is exhibited 

 by the young Amphioxus in the course of its development 

 (Willey), without any present physiological necessity being 

 able to explain it ; its two series of branchial clefts, the 

 projecting folds between which they are enclosed, which are 

 the first traces of the subbranchial cavity of the adult animal, 

 are formed on the right side of the body exclusively and mark 

 out the curves along which the torsion has been accomplished. 

 The application of the law of patrogony necessitates the 

 conclusion that the ancestors of Amphioxus passed through a 

 period at which they lived lying on the left side, with the 

 mouth in contact with the ground, and when they were 

 obliged, in order to ensure the regularity of their respiration, 

 to twist the branchial region of the body. This permanent 

 attitude has produced, in consequence of the constant contrac- 

 tion of certain muscles and the relaxation of those opposed to 

 them, an asymmetry of the body which has been transmitted 



