SENSE ORGANS. YASCtfLAR SYSTEM. 



307 



;tactile organs. The latter are joined to nervous expansions and 

 special integumentary appendages (tactile hairs), and are present 

 even in the parasitic Worms as papilla? of the outer skin connected 

 with nerves. Iri the free-living worms, these tactile organs fre- 

 quently take the form of filiform, tentacle-like appendages on the 

 head and segments (cirri). Auditory organs are not so generally 

 present, and are represented by auditory vesicles (otocysts) either 

 lying on the brain (some Turbellaria and JVemertinea), or on the 

 cesophageal ring (certain branchiate Worms among the Annelida). 

 The organs of sight are simple pigment spots in connection with 

 nerves (eye-spots), and may be provided with refractive bodies. The 

 ciliated pits of the Nemertinea have been regarded as organs of smell. 

 The cup-shaped organs of the Hirudinea and GepJiyrea are also sense 



organs. 



Br 



FIG. 21G. Section through a body segment of Eunice. Sr y 

 branchial appendages ; C, cirri ; P, parapodia with tuft 

 of bristles ; D, intestine ; N, nervous system. 



A blood vascular 

 system is wanting in 

 the Nemathelminthes, 

 the Rotifera, and the 

 Platyhelminthes with 

 the exception of the 

 Nemertinea. In 

 these cases, the nu- 

 tritive fluid passes 

 endosmotically into 

 the body parenchyma 

 or into the body cavi- 

 ty, and penetrates the 

 tissues as a clear chyle, sometimes containing cellular elements. In the 

 Nemertinea a blood vascular system is present, as also in the Gepliyrea 

 find Annelida. In the latter it obtains the highest development, and 

 may have the form of a completely closed vascular system provided 

 with pulsating trunks. In most cases a dorsal contractile longitudinal 

 trunk and a ventral vessel can be distinguished ; the t\vo being- 

 connected in each segment by arched transverse vessels, which are 

 sometimes pulsatile. Where a vascular system is present, the blood 

 does not always appear clear and colourless like the fluids of the 

 body cavity, but sometimes has a yellow, greenish, or more frequently 

 red colour, which is in some cases connected with the presence of 

 blood corpuscles. 



The function of respiration is usually performed by the general 

 external surface of the body. Among the Annelida, however, we 



