PLA TYHELMINTHES 



The position of the mouth in the diiferent species of Tue- 

 BELLAEIA Varies gi-eatly ; in some it is anterior, in others it is 

 ventral and median, and it may be nearly at the posterior end 

 of the body. True nematocysts, 

 such as are found in the Coelen- 

 TERATA, exist in the ectoderm of 

 a few genera, amongst others in 

 Microstoma. Some of the ecto- 

 dermic cells are also modified, and 

 form adhesive or glutinous areas 

 which make up for the want of 

 suckers. 



The pigment may be in the 

 ectodermal cells, or, as in 

 Mesostoma, in the parenchyma ; 

 Vortex viridis and Gonvoluta 

 Schultzii contain cells coloured 

 with chlorophyll and with starch 

 in them. These are probably 

 symbiotic algae. 



The arrangement of the 

 muscle fibres in Mesostoma is 

 very simple ; the diagonal layer 

 exists in many genera, and in 

 some of the Dendrocoels there 

 may be as many as six separate 

 layers. 



The disposition of the phar- 

 ynx and stomach in Turbellarians 

 is very various, and has served as 

 a basis for the classification of 

 the class. The muscular part of 

 the pharynx may be very much 

 enlarged, and capable of being 

 protruded through the mouth, 

 and acting as a powerful sucker. 



In one division of the Ehabdocoehda, the Acoela (Fig. 61), 

 the pharynx ends in the soHd parenchyma, into which the food 

 passes, and is there digested ; in the Ehabdocoela the alimentary 



Fig. 



62.— Plan of a Triolad Turbel- 

 larian. After Von Graff. 



1. Anterior. 

 ', 1". Posterior branches of alimentary- 

 canal. 

 5. Oviduct. 

 9. Tentacle. 

 10. Vas deferens. 

 2, 3, 4, 6, 7, 8, 11, 12, 13, as in Fig. 59. 



$. Female copulatory organs. 

 i $. Common sexual aperture. 



