138 INVERTEBRATE MORPHOLOGY. 



The excretory system differs from that of the lower orders 

 in that the two longitudinal uephridia open on the dorsal sur- 

 face of the body by numerous pores, which in Gunda corre- 

 spond in number with the intestinal diverticula and nerve- 

 commissures. The reproductive apparatus consists of nu- 

 merous testes (Fig. 73, t\ as in the Accela (arranged rnetameri- 

 cally in Gunda), whose ducts unite to vasa deferentia (vd) 

 uniting in the muscular iutromittent organ which projects into 

 the genital atrium. Two small ovaries (ov) occur in the ante- 

 rior end of the body, their large oviducts passing backwards to 

 unite in a muscular bursa copulatrix, and receiving at inter- 

 vals the secretion of numerous lateral diverticula which con- 

 stitute the vitellarium (vi). A pouch-like diverticulum of the 

 atrium serves as a uterus, and the single median orifice (p) of 

 the atrium lies near the posterior end of the body behind the 

 mouth-opening. 



5. Order Polycladea. 



The Polyclads are exclusively marine and assume various 

 forms, some being quite elongated while others are flat leaf- 

 like expansions. Compared with the members of the other 

 orders they may be said to be as a rule Jarge, though few 

 reach the length which has been mentioned for some land 

 Triclads. The mouth varies greatly in position, as in the 

 Rhabdoccels, and opens into a spacious pharyngeal pouch 

 containing a plicated pharynx (Fig. 7% pli). The intestine 

 consists of a central cavity, into which the pharynx opens at 

 its inner end and from which numerous branches (hence the 

 name of the order) pass off into the compact parenchyma, 

 where they branch and may anastomose with one another to 

 form a network. The nervous system presents a somewhat 

 similar condition, the bilobed brain (ce), usually situated near 

 the anterior end of the body, giving off a number, usualty six, 

 of nerve-cords which become lost in a wide-meshed network 

 ramifying through the body-tissues. Eyes are usually pres- 

 ent, frequently in enormous numbers, and furthermore in many 

 forms (Planocera] tentacles arise from the dorsal surface or 

 else from the margin near the anterior end of the body. As 



