XII. PHYLUM ANNELIDA 



A. PRELIMINARY STUDY OF NEREIS 



Nereis is an annelid living in the mud along the shores of the ocean. It is 

 a relatively simple annelid and therefore used here to illustrate certain points. 

 The dissection, however, will be done upon the earthworm, a more specialized 

 form, but one more easily dissected. 



i. External anatomy of Nereis. Place a specimen in a dissecting pan. 

 Compare general form and symmetry with animals previously studied. Is it 

 bilaterally symmetrical? The most striking feature of the body is its division 

 into a longitudinal series of rings, called segments, metameres, or somites. In this 

 respect Nereis shows a marked advance over Planaria and resembles the frog. 

 Each segment bears a pair of broad lateral outgrowths, the parapodia, by means 

 of which the animal swims. Distinguish anterior and posterior ends, dorsal 

 (rounded) and ventral (flattened) surfaces. 



a) Head: The first segment of the body is markedly differentiated as a 

 head, which bears a number of sense organs. The head segment consists of two 

 parts, a dorsal squarish projection, the prostominm, which does not extend to 

 the ventral side, and just behind this a complete, ringlike segment, the peri- 

 stomium, which surrounds the mouth. (In most specimens the large pharynx 

 will be found protruded from the mouth so as to extend some distance anterior 

 to the head. This should not be confused with the true head.) 



The sense organs of the head consist of tentacles, palps, and eyes. There are 

 two short terminal tentacles projecting from the middle of the anterior edge of 

 the prostomium. Lateral to these on each side is a thick jointed palp. On the 

 dorsal surface of the prostomium are four eyes, indistinct blackish spots situated 

 so as to form the four corners of a trapezoidal area. The two posterior eyes are 

 likely to be concealed under the anterior edge of the peristomium. The eyes of 

 Nereis are much more complex than those of Planaria and are provided with 

 lenses, so that the animal can probably distinguish objects in a dim way. The 

 peristomium bears four conspicuous, long, lateral tentacles, or cirri, on each side. 

 It is probable that tentacles and palps are organs of touch, contact, chemical 

 sense, etc. It is known that they are thickly strewn with sensory nerve cells. 



The head of Nereis therefore represents another step in advance in the series 

 of animal forms which we are considering in that it is more sharply differentiated 

 from the rest of the body than is the case in the coelenterates and flatworms, 

 and it is provided with a greater variety of sense organs and more complex and 

 sensitive ones. 



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