GASTROTRICHA 



623 



cells on the esophagus are designated as salivary in character 

 and other gland cells in a fringe at the beginning of the intestine 

 have been regarded as hepatic or liver cells. The food of the 

 Gastrotricha consists mainly of unicellular algaCj the tests of which 

 can often be recognized in the intestine. 



Six pairs of delicate longitudinal strands constitute the .entire 

 muscular system as there are neither circular nor oblique muscles. 

 One pair of muscles extends nearly 

 the eiltire length of the body; ^ 

 the others occupy only the ante- 

 rior or the posterior region meeting 

 near the center. They lie in the 

 body cavity which is devoid of 

 any special lining epithelium. The 

 head region of the body cavity 

 is almost completely filled by a ^ 

 saddle-shaped mass (Fig. g64,B) of 

 nerve cells dorsal and lateral to 

 the esophagus which constitutes 

 the brain. From it nerve fibers 

 go out to the anterior sense hairs 

 and lateral strands follow the 

 alimentary canal to the posterior 

 end of the body. Minute pigment 

 spots which may be designated as eyes certainly occur in some 

 species despite the doubts expressed by ZeHnka. 



Excretory organs are present in the form of a pair of lateral 

 much coiled tubes near the center of the body. The inner end of 

 each tube is closed by a long flame cell and the outer end opens 

 on the ventral surface near the median line and just behind the 

 center of the body. Just behind these coils, in the mature females, 

 lie the large eggs that mark the anterior limit of the simple ovary. 

 These eggs become so large that they extend over one-third to 

 one-half the length of the entire body and increase its normal 

 transverse diameter noticeably so as to modify greatly the form of 

 the gravid female. The large, oval eggs are laid on algal threads 

 or empty shells of other animals. When deposited they have a 



Fig. 964. Chaetonotus maximus. A , optical longitu- 

 dinal_ section through anterior tip of the body 

 showing mouth, oral bristles, frontal plate, and 

 esophagus. X 535. B, cross section through 

 posterior extremity of A, showing tripartite 

 esophagus and above it the saddle-shaped brain 

 while below are the two ventral cilian bands. 

 X 690. C, longitudinal section through the pos- 

 terior region of the body showing below the in- 

 testine, rectum, and anus; above in the ovary a 

 single ripe egg; on the upper surface fine 

 bristles, and at the posterior tip one of the toes 

 with its adhesive gland. X 430. (After Zelinka.) 



