SUMMARY OF IMPORTANT FACTS 237 



30. The digestive tract consists of an ectodermal oesophagus (stomach) 

 and a branching system of entodermal vessels. 



VERMES. 



A large number of forms above the Coelenterates are frequently grouped 

 as Vermes, but there is little agreement as to what smaller divisions shall be 

 included, some denying the existence of a natural group of worms, and separating 

 the groups as phyla. Others include not only the flat-, round- and segmented 

 worms, the Chstognaths and rotifers, but also the brachiopods, Polyzoa and 

 even the tunicates. Yet such is the variety of form, structure and development 

 that, no matter what limitation be accepted, it is impossible to frame a definition 

 which shall include all of the species commonly known as worms. In taking 

 the step from the diploblastic Ccelenterates to even the lowest 'worms' such 

 advances in structure are seen that a brief review of these is appropriate here. 



The worms are distinguished from the Ccelenterates by bilaterality, which 

 is seen in the internal structure, in even those cases (round worms) where it is 

 not visible on the exterior. There is also a higher degree of differentiation of 

 organs — the development of a ganglionic nervous system, excretory organs, and 

 frequently of a blood-vascular system. This advance is correlated with the 

 appearance of a true mesoderm, the layer from which (the nervous system 

 excepted) these organs and the muscles arise. Then there is the dermo-musadar 

 sac, the cause of the familiar 'worm-like' motions. This consists of an intimate 

 connexion of the skin with the underlying muscles (figs. 212, 240, 241). The 

 skin, a one-layered epithelium, ciliated or covered with a cuticle, rests on either 

 a structureless memhrana propria or on a cellular connective tissue, to which 

 the muscles are attached. Longitudinal muscles are always present, and fre- 

 quently circular as well, while in the parenchymatous worms diagonal, crossed 

 and dorsoventral fibres may occur. 



While certain worms (cestodes) lack an alimentary canal, or like many 

 nematodes, may have a blind, functionless gut, these conditions are the result 

 of parasitic habits. In the lower worms the digestive tract is like that of the 

 Ccelenterates, consisting of archenteron and stomodeum, proctodeum and anus 

 being absent. In most worms the tract is 'complete,' it being a tube with mouth 

 and vent. 



The digestive tract is either imbedded in the parenchyma and cannot be 

 dissected out (fig. 212), or it is surrounded by the body cavity (ccelom) which 

 separates it from the body wall (figs. 238, 241), the muscles of which are meso- 

 thelial in origin, in contrast to the mesenchymatous muscles of the flat worms. 

 In the flat worms the excretory organs are protonephridia, and similar organs 

 with solenocytes (p. 105) are common in the larva of the higher worms, being 

 often replaced in the adult by true nephridia. 



Here, too, a blood-vascular system first appears. Where it is lacking (flat 

 worms) its place may be taken by the branches of the digestive tract, or in higher 

 forms by the coelom with its albuminous fluids. The phylogenetic origin of 

 the circulatory system may be considered here. Two explanations of the 

 circulatory vessels have been advanced, (i) They are canals sepiarated from 

 the alimentary canal and developed farther independently; therefore they have 

 an entodermal epithelium. (2) They are spaces filled with albuminous fluid, 

 arising between entoderm and mesoderm (a modified form of this is that they 

 are remnants of the segmentation cavity); hence at first they had no epithelium 

 and obtained it later from the mesoderm. Lately the latter view is regarded as 

 the more probable, since most observers deny the existence of an epithelium in 

 the blood vessels of worms and other invertebrates. Then in many worms 



