202 AQUATIC MICROSCOPY FOR BEGINNERS. 



h. Worm with two small anterior pulsating hearts; 

 blood bright red. Tiibifex, lo. 



h. Worm without distinct hearts; dorsal vessel pul- 

 sating; blood red. Ndis, ii. 



I. Prist£na (Figs. 147, 148, 149). 



Body nearly cylindrical, transparent, frequently 

 very long, and often showing that it is preparing to 

 divide across the middle to produce two worms. In 

 these cases the proboscis of the new worm becomes 

 conspicuous at the center of the long body. The 

 mouth is near the base of the snout like prolongation, 

 this narrow extension of the upper lip varying much 

 in length in the various species. The one represented 

 in Fig. 147 belongs to a common form in the writer's 

 locality, and is unusually long. 



Fig. 147. — Snout of 

 a Pristfna. 



Fig. 148. — Posterior extrem- 

 ity of a Pristina. 



Fig. 149. — Posterior ex- 

 tremity of a Pristina. 



The posterior extremity is commonly nearly as 

 shown in Fig. 148, and surrounded by many short 

 stiff hairs, it being the tail-end of the PrisHna whose 

 proboscis is shown in Fig. 147. Occasionally this 

 part has three long trailing appendages, as in Fig. 

 149. The blood is usually red. 



The bristles are long and fine, and are often accom- 

 panied by one or more short, nearly straight, rudimen- 

 tary spines. The podal spines are in two rows on the 



