252 ELEMENTARY BIOLOGY. [CHAP. 



removal from the body. Examination of a specimen 

 in which this is the case shows that but one-third 

 of the seta projects beyond the body-wall when at 

 rest (Cf. Sect. H. 3). 



C. The body-cavity and general disposition of the viscera. 



Pin a large worm down under water dorsal side upper- 

 most, and slit up the body-wall along the dorsal middle line. 

 Pin back the two flaps and note 



a. The presence of a spacious body or perivisceral 

 cavity. 



b. The alimentary canal, a straight tube running the 

 whole length of the body. 



The yellowish-brown tissue investing its walls, ren- 

 ders it especially conspicuous for all but the anterior 

 20 segments. 



c. The mesenteric septa, a series of delicate transverse 

 membranous partitions, coincident with the somitic 

 constrictions (Cf. Sect. A. 2. a}. They attach 

 the alimentary canal to the body-wall. 



d. The segmental organs or nephridia, whitish fluffy- 

 looking appendages, one pair to each mesenteric 

 septum. Examine under a lens, and note that each 

 consists of a delicate convoluted tube, often of a 

 dead-white colour owing to the presence of ex- 

 cretory matter. 



e. The reproductive organs. Conspicuous among these 

 will now be seen the seminal vesicles ; three pairs of 

 large white bodies, overlying the alimentary canal 

 in segments 9 to 12. 



The relative size of these is variable. (Cf. p. 

 246.) The reproductive organs are restricted to this 



