256 M. Edmond Perrier o?i the 



number of them, the anterior segments, for instance (Amphi- 

 CTENID.E, Ampharetid^e, Terebellid.e, Amphioxus). The 

 nephridia of the Nemertines are also confined to the anterior 

 region of the body, and eventually open into a collecting duct 

 on each side ; herein they seem to present a further resem- 

 blance to those of the Vertebrates : but analogous conditions 

 exist in the case of the Annelid Worms {Loimia medusa, 

 Lanice conchylega) ; in certain Lumbricidse [Octochcetus, Peri- 

 chceta) and some Hirudineans (Pontobdella) the nephridia 

 form throughout the body but one and the same system of 

 reticulated tubes. Moreover, in the Annelid Worms the 

 nephridia almost always place themselves more or less at the 

 service of the genital apparatus, as in the case of the Verte- 

 brates. In the Earthworms (Lumbricimorpha) there even 

 occurs a doubling of the renal apparatus and of the excretory 

 apparatus of the genital glands, the analogies between which 

 and that which is observed in the case of the Vertebrates 

 (Wolffian and Miillerian ducts) I set forth as early as 1881 *. 

 The advantage therefore rests with the Annelid Worms, of 

 which the Nemertines are in all probability nothing but 

 highly modified forms. The Annelid Worms being hence- 

 forth alone concerned, it remains for us to explain how it has 

 been possible for them to be the point of departure for the 

 organic conditions which are peculiar to the Vertebrates. 



(6) Notochord. — If it has been possible to compare for one 

 moment the endodermic diverticulum of the proboscis of 

 Balanoglossus, or even the proboscis of the Nemertines with 

 the notochord of the embryos of Vertebrates and of the Tuni- 

 cates, it is in default of having rigorously defined the con- 

 ditions which have brought about the formation of the 

 notochord and the necessary relations which result from this 

 formation. The embryogeny of Amphioxus, like that of the 

 Tunicates, shows in fact that the notochord is originally not 

 a cord ot cells, but the entire region of the endoderm included 

 between the rudiment of the nervous system and the two 

 rudiments of the mesoderm. These three rudiments are 

 three regions of active development, which are only able to 

 derive the reserve of nutriment which is necessary for them 

 from the endodermic area comprised between them. As a 

 result of this the elements belonging to the area thus circum- 

 scribed empty themselves, become vacuolate and to a certain 

 extent mortified ; the rest of the endoderm, on the contrary, 

 continues to develop, glides in consequence underneath the 

 inert area and eliminates it from the endoderm, and it is this 



* 'Lea Colonies animales,' pp, 677 and 684. 



