864 



kart and Pagenstecker proved it. Our figure taken from the 



dinm, and the planarian-like Nemertes with the eye- specks (Fig. 

 170, e), growing in it. How the worm originates in the body of 

 the Pilidium. and how the hitter arises, have lately, been fully shown 

 by Metsehnikoff, and to his memoir we are indebted for the 

 strange history of the alternation of generations in these worms. 



He followed the development of the Pilidium from the egg, 

 which undergoes total segmentation, leaving a segmentation-cav- 

 ity. The next occurrence is the separation of a one-layered cili- 

 ated blastoderm, the ectoderm, which invaginates, forming the 

 primitive digestive cavity, from which the stomach and oesophagus 

 are formed. The larva is now helmet-shaped, ciliated, with a long 

 lash (flagellum) attached to the posterior end of the body. 



After swimming about on the surface of tire sea awhile, the 

 Nemertes begins to grow out from near the oesophagus of the 

 Pilidium. On each side of the base of the velum (v) of the Pilid- 

 ium appear two thickenings of the skin, one pair in front, the other 

 behind ; these thickenings push inwards, and are the germs of the 

 anterior and posterior end of the future worm. The anterior pair 

 become larger than the posterior ; the part of the disk next to the 

 cesophagus thickens ; at the same time the alimentary canal of 

 the Pilidium grows smaller and only a narrow slit remains. The 

 disks now divide into two layers, the outer much thicker than 

 the inner. A new structure now arises, a pair of vesicles near the 

 hinder pair of disks ; these are the "lateral organs" of the future 

 worm. Soon the anterior pair of disks unite and the head of the 

 worm is soon formed, when the elliptical outline of the flat worm 

 is indicated, and appears somewhat as in Fig. 170 (», intestine 

 of the worm). The yolk mass, with the alimentary canal of the 

 Pilidium, is taken bodily into the interior of the Nemertes, the 

 Pilidium skin falls off, and the worm seeks the bottom. 



Metsehnikoff discovered five other species of Pilidium, and 

 thinks this mode of development is not an uncommon occurrence. 

 This manner of development is directly comparable with that of 

 the echinoderm from the Pluteus. 



To show the wide range of metamorphosis existing in the Ne- 

 merteans, we may cite the case of a Nareda studied by Mr. A. 

 Agassiz, and whose early stages are like those of the higher 

 Annelides, in fact so much so that Milne-Edwards and Claparede 



