EXPLANATION OF PLATES X. AND XL 437 



Fig. 12. — An older larva of the Ampbioxus. On both sides of tho 

 medullary tube (m) and of the notochord (e/() a longitudinal row of muscle- 

 plates (mp) is visible ; these mark the embryonic vertebrae, or metamera. 

 An organ of sense has developed in front {ss). The w^all of the intestine 

 (d) is much thicker below on the ventral side (du) than above on the dorsal 

 side (do). The anterior part of the intestinal canal widens into the gill- 

 body. 



Fig. 13. — Transverse section through a developed Amphioxus (Fig. 15) 

 a little behind the centre of the body. Above the intestinal tube (d) is the 

 dorsal blood-vessel, or main artery (t), and below it the ventral blood-vessel, 

 or the intestinal vein (v). At the inner wall of the gill-cavity (c) lie the 

 ovaries (e), and outside these the side canals (it). The dorsal muscles (r) 

 are divided into several parts by inter-muscular ligaments (mi) ; /, dorsal 

 fin. 



Plate XI. — Structure of the Ascidian, of the Amphioxus, and of the 



Lakva of the Petromyzon. 



For the sake of comparison, all the three animals are placed in the same 

 position and are represented of the same size. The view is from the left 

 side. The head extremity is turned upward, the tail downward ; the dorsal 

 side to the right, the ventral side to the left. The enveloping membrane is 

 removed from the left side of the body, to show tho inner organization with 

 the organs in their natural position. 



Fig. 14. — A simple Ascidian (Monascidia), magnified six times. 



Fig. 15. — A developed Amphioxus (magnified four times). 



For the sake of giving a more distinct view, tho Amphioxus in Fig. 15 is 

 drawn about twice its actual breadth. In i-eality, its breadth amounts to 

 but half of the length as represented here. 



Fig. 16. — Young larva of a lamprey {Pctromyzon Pla7ieri), eleven days 

 after emerging from the egg, magnified 45 times. (After Max Schultzc.) 

 The larva of the lamprey, which undergoes a peculiar transformation at a 

 later period, was formerly considered as a distinct species under the name of 

 Ammoccetes. 



The meaning of the letters is the same in all the figures. 



