134 INVEBTEBRATE MORPHOLOGT. 



the anterior or the posterior end of the body, and opens into 

 a pharyngeal pouch, whose walls are thickened by muscle-fibres 

 in such a way as to form a somewhat bulbous mass sharply 

 marked off from the parenchyma which surrounds it. In 

 Monotus, however, the pharynx is more developed, projecting 

 as a strong circular fold into the pharyngeal pouch and form- 

 ing what is termed a plicated pharynx. This at its inner 

 extremity communicates with the sac-like intestine, usually 

 quite simple but occasionally somewhat pouched, and ter- 

 minating, as in all the Turbellaria, blindly. 



The nervous system consists of a bilobed ganglionic brain- 

 mass from which pass backwards two nerve-cords which may 

 (Monotus) or may not present transverse anastomosing 

 branches, and in addition a number of smaller branches pass 

 forward to be distributed to the anterior end of the body. 

 Eyes, consisting of pigment-spots seated upon the brain, are 

 frequently present, and in Monotus an otocyst is found, while 

 lateral ciliated depressions on each side of the head occur in 

 Plagiostoma. 



The excretory system is present, but presents no notable 

 departures from the typical arrangement. As regards the 

 reproductive organs, the testes resemble those of the Acoela, 

 but the ovaries are comparatively small and the separate 

 vitellaria are large and sometimes branched, opening into a 

 cavity, the genital atrium, copimon to them, the oviducts 

 and the intromittent organ, and communicating with the ex- 

 terior by a single median pore situated near the posterior end 

 of the body. In a few forms the vitellaria are not differen- 

 tiated from the ovaries, presenting a condition similar to that 

 found in the Acoela. 



3. Order Rhabdocoela. 



The Ehabdocoela are found both in fresh and salt water and 

 are usually small. They possess a distinct tubular digestive 

 tract (Fig. 69, G, d) without lateral pouches or branches, but 

 the principal characteristic lies in the presence in the paren- 

 chyma of large spaces resembling the ccelomic cavities of 

 higher types, a feature not repeated in any other Turbellaria. 



