THE GROUPS OF ANIMALS 



275 



Class V. Arachnida. Arthropods with tracheae, book lungs or book gills and no 

 antennae. Spiders, mites, scorpions, king crabs. (Fig. 240.) 



Invertebrate Groups of Uncertain Position. — Certain groups of inver- 

 tebrates have not been assigned a definite relation to other groups. 

 Opinion differs so widely as to their affinities that they may well be 

 kept out of the classification for the present. 



Mesozoa. Parasites apparently intermediate between the protozoa and metazoa. 

 Not improbably degenerate relatives of the flatworms. , ;,». 



Nemertinea. Terrestrial, fresh-water, and marine animals resembling flatworms 

 but with a proboscis, blood-vascular system, and alimentary canal with two openings. 



Nematomorpha. Long threadlike animals with the body cavity lined with 

 epithelium, a pharyngeal nerve ring, and a single ventral nerve cord. 



Acanthocephala. Parasitic worms with spiny proboscis, a complex reproductive 

 system, and no alimentary canal. (Fig. 241.) 



Fig. 242. — Arrowworm, Sagitta. 



Fig. 241. — Echinorhynchus, one of the 

 Acanthocephala. 



Chaetognatha. Marine invertebrates with a distinct coelom, alimentary canal, 

 nervous system, and two eyes. Arrowworm. (Fig. 242.) 



Ctenophora. Triploblastic animals; symmetry partly radial, partly bilateral; 

 eight rows of vibratile plates radially arranged. Sea walnuts or comb jellies. (Fig. 

 243.) 



Fig. 243. 



ctenophore. 

 Cleave.) 



{From Van 



Fig. 244. — A rotifer. 

 {From Whitney.) 



Rotifera. Invertebrates with a head provided with cilia, usually a cylindrical or 

 conical body often with a shell-like covering, and a tail or foot, bifurcated at the 

 tip where it is provided with a cement gland. (Fig. 244.) 



Bryozoa. Mostly colonial invertebrates resembling hydroids in form, with dis- 

 tinct coelom, and with digestive tract bent in the form of a letter U. (Fig. 140.) 



Phoronidea. A single genus of wormlike animals having tentacles and living in 

 membranous tubes in the sand. 



