THE PHYLA ASCHELMINTHES AND NEMERTEA 225 



a dorsal antenno (Fig. 12.3), a short projection rich in sensory endings. 

 Eyespots are light-sensitive cells containing pigment that screens out the 

 light except from one direction. These are found in many rotifers em- 

 bedded in the brain, on the wheel-organ or on the rostrum. 



In addition to all of the complex structures found in these tiny 

 animals, they have a cavity between the body wall and the digestive 

 tract. The pharynx and bladder, which have muscles, are formed as 

 ectodermal invaginations during development. The rest of the digestive 

 tract is a simple gastiodermis, without mesoderm, so that the body 

 cavity is a true pseudocoel, lying between endoderm and mesoderm. 



84. Reproduction in Rotifers 



The creeping rotifers (including Philodina) are parthenogenetic; 

 young are produced from eggs that have not been fertilized by sperm. 

 In oogenesis, the meiotic process is much modified, with the result that 

 the eggs remain diploid. Such eggs hatch in a day or two and mature 

 within a week into adults, all of which are female. Each adult produces 

 only from 10 to 50 eggs. 



Males are occasionally found in the other gioups of rotifers, but 

 much of the reproduction is exclusively by parthenogenesis. Under cer- 

 tain environmental conditions the new generation of females matures 

 as somewhat different organisms. The eggs they produce are smaller, 

 and follow through the normal meiotic process to become haploid. The 

 first of these eggs are laid and hatch quickly as males. The males are 

 haploid, often remain very small, and mature rapidly. They mate only 

 with members of their mother generation, the females that are producing 

 small haploid eggs. The small eggs that are fertilized, restoring dip- 

 loidy, are retained until they become very large, when they are laid in a 

 heavy shell, usually as a resting egg for overwintering. These resting 

 eggs later hatch into females that produce only female offspring, com- 

 pleting the reproductive cycle. 



85. Cell Constancy 



Associated with rotifers are several interesting phenomena, one of 

 which is cell constancy. In a given species, each part of the body is 

 made of a precise number of cells arranged in a fixed pattern. Many of 

 the body parts are syncytial (cell boundaries disappear), but it is evident 

 from the number and positions of the nuclei that cell constancy is main- 

 tained. The total number of nuclei in the rotifers studied ranges from 

 900 to 1000. The exact number in each organ has been counted for sev- 

 eral species. These numbers are fixed during embryologic development, 

 and mitosis then stops completely. Even the eggs that the female will 

 produce after maturity are all present early in development. 



It has been impossible to induce mitosis in adult rotifers experi- 

 mentally. If a piece of the body containing nuclei is removed, no re- 

 generation takes place. Often the wound does not heal over and the 

 individual dies. Young rotifers are able to replace bits of cytoplasm, 



