260 



CCELHELMINTHES 



stomach), glandular stomach, and intestine; all except the mastax ciliated. 

 The mastax bears two chitinous jaws (trophi), which in life are in constant 

 motion and comminute the food. Above the oesophagus is the cerebral ganglion 

 with which simple eyes and peculiar sense organs, the cervical tentacles, are 

 frequently connected. The usually single ovary and the paired protonephridia 

 empty into the posterior part of the alimentary canal, which thus becomes a 

 cloaca. The males are much rarer and smaller and have a much simpler 

 structure (fig. 239, 5). Usually the alimentary tract is reduced to a solid cord 

 in which the testes are imbedded. 



PIG. 239. Brachionus urceolaris. A, female with four eggs in various stages; B, 

 male; C, 'flame' from protonephridia, greatly enlarged; b, urinary bladder; c, cloacal 

 opening; (/, gastric glands; g, ganglion, with eye; /;, testis; k, mastax; m, stomach; o, 

 ovary; p, penis; /, tentacle; iv, protonephridia. 



The Rotifers have large winter eggs enclosed in a thick shell and smaller 

 thin-shelled summer eggs. The latter develop parthenogenetically and by their 

 numbers and rapid growth aid in the distribution of the species. The winter 

 eggs require fertilization, and have a long resting period, thus serving to tide 

 over periods of cold or drought. The adults can withstand a certain amount 

 of desiccation; and often occur in damp moss or in eave troughs in a sort of 

 sleep from which they are awakened by water. 



In structure the Rotifiers are much like the trochophore larva; of annelids 

 and molluscs to be described later. They are primitive forms, connected with 

 the ancestors of these groups, and also, as sho\vn by nervous system and excretory 

 organs, with the flatworms as well. Most species are cosmopolitan and in- 

 habitants of fresh water. Near the Rotifera may be placed the fresh-water 

 GASTROTRICHA (Ichtliydiiim* Clurttmotits*) and the marine ECHINO- 

 DERID^E, forms which are little understood. 



PHYLUM VI. CCELHELMINTHES. 



The Ccelhelminthes are distinguished from all the forms which have 

 gone before by a body cavity, separating the outer body wall from the 



