86 LABORATORY STUDIES IN ZOOLOGY 



probably that of providing a sense subserving as taste. On tlie 

 dorsal surface of the prostomium arc four indistinct grey spots, 

 each marking the location of a light-perceiving organ. The 

 peristomium bears four pairs of lateral peristomial tentacles 

 or cirri. 



Make a four-inch drawing of the dor so -lateral aspect of the 

 left side of the head and first segment of the body. Label the 

 parts described above. 



Body and Tail Metameres. — The large number of segments 

 caudad of those already mentioned as constituting the head may 

 be regarded as the body metameres, excepting the last which is 

 called the tail segment. Note the similarity between the body 

 segments. Internally, parts will be found repeated in successive 

 segments. 



The posterior or tail segment differs from the others in the 

 modification of the parapodia into a pair of tactile processes or 

 cirri. In this segment the anus is located. 



Parapodium. — Compare the parapodia at different parts of 

 the body for similarities or differences. Study a parapodium 

 which has been removed and mounted in balsam, or cut off one 

 with the scissors, mount in water, and cover with a glass slip. 

 Employ a dissecting lens or the low power of the compound 

 microscope. Compare the parapodium studied in this way with 

 those still attached to the animal and determine which is its 

 dorsal and which its ventral edge. 



These lateral outgrowths of the body wall serve both as respira- 

 tory and locomotor organs. A parapodium can be divided into 

 two distinct portions, a dorsal called the notopodium and a ven- 

 tral called the neuropodimn, each of which is stiffened by an 

 internal chitinous supporting rod, called the aciculum or bristle. 

 Find the two acicula. The large dorsal lobe of the notopodium 

 is a respiratory organ, that is, a gill, containing a network of 

 blood vessels which can easily be seen in life. Attached to its 

 dorsal edge is a slender vibratile sense organ, the dorsal cirrus. 

 The ventral portion of the notopodium comprises two lobes the 

 smaller of which bears fine bristles or seta. The neuropodium 

 likewise comprises two lobes, the notched one of which is setas 

 bearing. Beneath these lobes on the ventral margin of the neuro- 

 podium is a ventral cirrus. 



Make a three-inch outline drawing of a parapodium, labeHng 

 the structures described above. 



