. Fresh-water biology. Freshwater biology. 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-

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. Fresh-water biology. Freshwater biology. 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 mout