DEVELOPMENT OF THE ANOPLA. 



121 



body corresponding to the mouth (*) and future digestive tract, and the usual long tuft of cilia. 

 The involution is next differentiated into an oesophagus and csecal stomach, the lining membrane 

 being furnished with cilia and the wall with cellulo-granular elements (b). About this time the 



Pig. 7 



wmmm 



~Pylidiwm, showing the young Nemertean and most of its internal organs. 



Pylidium leaves the egg, and swims freely in the water. The second stage is the formation of 

 the Nemertean in the interior of the Pylidium. The first step towards this end consists in the 

 appearance of four round thickenings of the skin (b, c, e 1 , e 3 ), two larger in front of the lobes of the 

 helmet-like Pylidium, and two behind, — the four corresponding to the " suckers " of J. Miiller. 

 The anterior pair soon increase in size and become divided into a thicker and a thinner half, the 

 former, moreover, making two folds. A " Seitenorgane" (x) (cephalic sac) appears in front of 

 each of the posterior processes (e 2 ), which grow into two elongated vesicles, each with a thicker 

 and a thinner portion. A commissure and the proboscis (rh) develop anteriorly, and other 

 changes ensue both there and in the posterior processes ; part of the latter investing the stomach, 

 the thicker fold being directed towards the mouth of the Pylidium, while the thinner, coloured 

 somewhat brownish, becomes converted into a very fine membrane, which forms a border to the 

 thicker portion. These two processes (anterior and posterior), which are separated only by the 

 utricle (Blaschen), become subsequently more closely arranged ; so that the Nemertean 

 embryo forms a semicircular mass. Further changes occur in the anterior and posterior processes, 

 and the various parts of the Nemertean become differentiated. A membranous envelope, am 

 (Amnion), is developed, in which currents are caused by the ciliated coating of the young worm. 

 The ganglia, the ducts of the cephalic sacs, and a caudal style also appear, and the oesophagus and 

 digestive sac assume a Nemertean type, the body of the young animal closing round the latter 

 cavity. Finally, an almost complete young worm is found in the interior of its envelope (wood- 



16 



