142 



INVERTEBRATE ZOOLOGY 



the digestive organs which occur as degenerate strings of tissue, 

 near the posterior region, to which the male gonad is attached. 

 A special copulatory organ in the form of a protrusible cirrus 

 is often present. 



Rotifers as Experimental Animals. — Because of the readiness 

 with which they may be reared in cultures, rotifers have been 

 widely used in the study of problems concerned with the deter- 

 mination of sex. For instance, in some 

 species the ratio of males to females may be 

 controlled directly by regulation of the 

 temperature or by the type of food given to 

 the parthenogenetic females. 



Nuclear Constancy. — For a number of 

 rotifers, it has been shown that each organ 

 is built of a fixed number of cells or at least 

 contains a constant number of nuclei. An 

 especially thorough study along this line has 

 been published by Eric Martini for Hyda- 

 Una senta, each individual of which contains 

 a total of 959 nuclei distributed in fixed 

 numbers through the various organs and 

 tissues of the body. 



Class Gastrotricha 



Though the Gastrotricha are here included 

 as a class along with the Rotifera under the 

 phylum Trochelminthes, their relationships 

 with the rotifers are far from firmly estab- 

 lished. Some zoologists maintain that the 

 Gastrotricha are more directly related to the 

 Nematoda. They are rarely more than 0.5 

 mm. in size and though they occur relatively 

 ovary. X400 {After frequently in protozoan and rotifer cul- 



Zelinka, from \\ ard and 



Whipple's Fre^h-water turcs, their Small sizc and rapid movements 

 Biology, reprinted by fgnder closc examination difficult. Though 



permission of John 



Wiley and Sons, Inc.). widely distributed, they are restricted to 

 fresh water. 

 In most instances, there is a head set off from the body proper 

 by a slight constriction. The body is flattened on the ventral 

 surface and convex on the dorsal. The ventral surface is 

 furnished with two longitudinal bands of cilia near the median 



Fig. 73. — Chaeton- 

 otus maximus, one of 

 the Gastrotricha, in 

 ventral view. Ex, kid- 

 ney; M, muscles; B, 

 brain; E, egg; O, esoph- 

 agus; /, intestine; Ov, 



