3.™ 



It should also be remembered that among the worms are many 

 synthetic types which, as regards some organs, remind ns of other 

 groups of animals. For example the Rotifers recall the lower 

 Crustacea, and are by some naturalists regarded as such; the 

 Planarians have been considered by Girard, as mollusks, the 

 Polyzoa and Brachiopods are still regarded as mollusks by emi- 

 nent naturalists, and there are very few who do not place the 

 Tunicates among the latter. On the other hand the Echinoderms 

 are regarded as worms by some, and Amphioxus has been called 

 a worm. Indeed if any one has any prejudices regarding fixed 

 types in nature, and would learn how regardless of preconceived 

 zoological systems the actual state of our knowledge of the lower 

 animals must lead one to become, let him study the animals now 

 placed among the " Worms." 



Leaving out of consideration the lowest forms, almost without 

 organs, and many parasitic forms, as 

 are bilateral, segmented animals with t 

 arate or united by commissures, and resting on the floor of the 

 body under the alimentary canal, which usually (when present) 

 passes directly through the middle of the body. There is in the 

 Annelides a dorsal and ventral blood-vessel, the circulatory appar- 

 atus being closed and more highly developed than in the Crustacea 

 and Insects, Limulus excepted. In the lower worms (Platyel- 

 minths, Nematelminths, Acanthocephali and Rotatoria) there is a 

 complicated system of excretory tubes, thought by some anato- 

 mists to be analogous with the water-vascular system of Radi- 



The organs of locomotion are, when present, simple bristles or 

 prolongations of the walls of the body forming paddle-like flaps. 



We are now concerned with tracing the mode of development 

 of some of the typical forms belonging to the different subdivis- 

 ions, the general relations of which may be seen in the following 

 tabular view, which is taken from Gegenbaur's "Principles of 

 Comparative Anatomy," with the addition of the Brachiopoda, 

 which he still retains among the Mollusca. The Onychophora, 

 represented by Peripatus, are also omitted, as since the publica- 

 by the re- 



