ANIMAL METAMORPHOSIS. 93 



elongates itself into a long, sausage-shaped body, develops a series 

 of feathery tentacles round the mouth, and five rows of tube-feet 

 along the sides of the body, by means of ^Yhich it draws itself 

 along the bottom of the sea. 



The changes which take place in the members of the next 

 sub-kingdom, the Scokcida^ or worms, are, in many respects, the 

 most curious and interesting in the animal kingdom. 



The simplest worms with which we are acquainted are the 

 Turbellarians. They are small, ciliated, flattened, soft bodies, 

 mostly aquatic in their habits, which glide along with a slug-like 

 motion over wet surfaces, or swim by the vibration of their cilia. 

 The egg of the Turbellarian gives rise to a larva, totally unlike the 

 parent, from which the adult is developed in a manner closely 

 analogous to that of the Echinodermata. This early form is often 

 a small, helmet-shaped organism, ciliated on the edges and side- 

 lobes, with a long flagellum attached like a plume to the summit 

 of the helmet. It was described by Johann Miiller, under the 

 name of Pilidium (Fig. 8). After swimming about for a time, a 

 worm-like body forms on the side of the larva ; eventually, it grows 

 round the alimentary canal, which it appropriates, and detaches 

 itself from the Pilidium^ and develops into the perfect adult. 



Many species of worms are parasitic, and not only undergo 

 various metamorphoses in the processes of their development, but 

 are also obliged in each stage to become the tenants of a different 

 host, in which alone that particular stage can be carried out. The 

 tape-worms and the cystic or bladder-worms were formerly sup- 

 posed to be distinct animals, but it is now known that the latter 

 are merely immature forms of the former. The egg of the tape- 

 worm, on reaching the digestive organs of a suitable animal, sets 

 free a little oval body, armed in front with hooks or boring-spines, 

 by which it travels through the tissues till it reaches a suitable site, 

 where it anchors, and developes from its hinder end a kind of 

 bladder filled with fluid. It is now a cysticercus, or bladder-worm 

 (Fig. 9), and in this condition may remain stationary for any 

 length of time, but for its further development it must be intro- 

 duced into another host. If, then, the flesh of an animal contain- 

 ing such cystic worms be eaten by another (of a suitable species), 

 the young tape-worm is liberated from its cyst ; attaches itself by 



