GENERAL ZOOLOGY 



Palp 



Prostomial 

 tentacles 



Peristomial 

 tentacles 



Yolk- 

 filled 

 endoderm 

 cells 



Zone of 

 growth 



Peristomial 

 tentacles 



Anal cirri 



Peristomium 



Pharynx 

 and jaws 



Midgut 



Hindgut 



Zone of 

 growth 



Fig. 14.3. Earlv developmental stashes of Nereis dumenln. The earliest free-swimmins; staa;e 

 in this form is not a trochophore (cf. Polygordius, Fig. 14.17, p. 418) but a larva with a head 

 and the beginnings of segmentation. In subsequent stages, note the succe.ssive addition of 

 segments at the posterior end, and the progressive modification of the first segment into the 

 peristomium (cf Fig. 14.2). (Redrawn from F. Hempelmann, 1911, ^oolng^icn, vol. 25.) 



in the thin dorsal lobes of the parapodia; these dorsal lobes serve as gills. 

 The movement of blood within the vascular system is effected by peristaltic 

 contractions of the larger vessels, particularly of the dorsal vessel. In the 

 living worm, pulsations can be seen passing from posterior to anterior in 

 this vessel. The blood is colored red by the respiratory pigment hemoglobin, 

 which in these worms is dissolved in the plasma, not contained in blood cells as 

 in a vertebrate (see p. 63). The excretory organs are paired nephridia, found 

 in all somites except those near the ends of the body. Each nephridium is a 

 tube, opening at one end into the coelom and at the other end passing 

 through the body wall and opening to the exterior. The inner end bears a 

 ciliated funnel, the nephridiostome, and the tube itself is lined in part by 

 ciliated epithelium. It is believed that waste products of metabolism are 

 collected from the coelomic fluid by these organs and so eliminated from the 

 body. 



The nervous system represents the typical annelid plan, consisting of a pair 

 of ganglia, the brain, located dorsally in the head region and connected 

 around the pharynx with the first of a series of ganglia making up the ventral 

 nerve cord. Nerves and nerve fibers radiate from the ganglia to innervate 

 muscles and sense organs and also to connect with parts of a subepidermal 



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