188 METAZOAN PHYLA 



220. Phylum Chaetognatha.— The group Chaetognatha (ke t6g'- 

 nath a; G., chaite, horse's mane, and gnathos, jaw) has been variously 

 placed in the animal kingdom; by some it is considered allied to the 

 nemathelminths and by others to the annehds. By still others it has been 

 ranked as a phylum. It is a small group containing the arrow worms, 

 the most familiar of which belong to the genus Sagitta (Fig. 98). These 

 are small, straight-bodied, and exceedingly transparent worms with a 

 true coelom, an alimentary canal, and the body divided into three parts, 

 which do not seem to be metameres. There are supraesophageal and 

 ventral gangha, two eyespots, and other sensory structures. Arrow 

 worms have neither vascular nor excretory systems. They swim about 

 on the surface of the sea, being present at times in very large numbers. 



221. Phylum Rotifera.— The phylum Rotifera (ro tif er a; L., rota, 

 wheel, and ferre, to bear) includes small, aquatic animals, many of them 



^an^//or> Ovary Anus vesicle 



Fig. 98. — Sagitta hexaptera D'Orbigny. Ventral \'iew. {From Lang, "Text-book of Com- 

 parative Anatomy," after 0. Hertwig.) X 20. 



not larger than protozoans. They are often encountered abundantly in 

 waters in which Protozoa also abound. Rotifers can be recognized by 

 the possession on the anterior end of ciha which in some cases are grouped 

 in circles. Such circular groups, owing to waves of motion which pass 

 rhythmically around the circle of ciha, produce when in action the effect 

 of revolving wheels (Fig. 99). Moreover, if the internal structure is 

 examined it at once reveals a complexity belonging not only to a metazoan 

 animal but also to one which is triploblastic and bilateraUy symmetrical 

 and which possesses organs and systems. Owing to the presence of the 

 rings of cilia on the head, these forms have often been called Trochel- 

 minthes (trok hel min' thez; G., trochos, wheel, and helminthos, worm). 



The bodies of rotifers are divided into three regions, which are a head, 

 a trunk, and a foot, the last forming the pointed posterior end. The 

 foot is frequently bifurcated and may assist in locomotion by serving to 

 kick the animal along (Fig. 100). It possesses a cement gland, the secre- 

 tion of which affords a means of attachment in the fixed forms. The 

 body is covered by a cuticula. The cilia are the principal means of 

 locomotion and also serve to create a current of water which brings food 

 to the mouth. There is a large body cavity which is like that of Nema- 

 thelminthes and therefore not strictly comparable with the coelom of 

 higher forms. It contains the alimentary canal, and in its walls are the 

 gonads and flame cells. The alimentary canal opens into a cloaca near 



