204 THE ANATOMY OF INVERTEBRATED ANIMALS. 



In the other somites the notopodial appendage is a large, 

 thin, oval plate — the elytron (Fig. 51, G, c). It is attached 

 by a thick peduncle, and has its long axis directed obliquely 

 outward and backward. The surface of the elytron (Fig. 52, 

 A) is covered with an ornamentation of larger or smaller 

 tubercular prominences, granulated and ridged upon their 

 surface. A part of the inner and anterior edge of each ely- 

 tron overlaps or is overlapped by its fellows for a certain ex- 

 tent of its circumference, which is so far smooth, but in the 

 rest of its extent it is fringed with coarse brownish filaments 

 or Jimbrice, which arise from the upper surface just within the 

 edge, and are obviously outgrowths of the same order as the 

 tubercles. 



Such is the structure of one of the middle somites of 

 Polyn'de squamata. The anterior and posterior somites, with 

 the exception of the first and second, present only minor dif- 

 ferences, as in the proportion of the setag, or in the figure of 

 the elytra. The first somite, which contains the mouth, is the 

 peristomium (" Mund-Segment " of Grube). The parapodia 

 of this somite are narrow and elongated (Fig. 53, J3, (7, m) ; 

 they are obscurely divided at their extremity into a rudimen- 

 tary neuropodium and notopodium, and give attachment to a 

 pair of large peristomial cirri (c r c) (" cirrhes tentaculaires," 

 Audouin and Milne-Edwards ; " Fiihler-cirren," Grube), of 

 the same structure as the notopodial cirri, which stretch for- 

 ward by the sides of the mouth. 



The apex of a single small aciculum issues rather above 

 the point of division of the peristomial parapodium, and two 

 minute curved setee accompany it. These have been generally 

 overlooked ; ' but they seem to demonstrate, in a very inter- 

 esting manner, the nature of the appendages of the peristo- 

 mial segment. 



The second somite differs from the rest only in the great 

 elongation of its neuropodial cirrus, which is directed forward 

 and applied against the mouth. 



The peristomium and the prsestomium together are ordi* 

 narily confounded under the common term of " head." The 

 latter (Fig. 53, J5, (7, I) is an oval segment flattened superior- 

 ly, placed altogether in front of and above the mouth, pre- 

 senting on its postero -lateral edges four dark spots, the eyes, 

 and possessing five cirriform appendages, two pairs and a 



1 At least, in the descriptions of the adult Polynoe. They are particularly 

 mentioned, however, by Max Miiller in his valuable paper, " Ueber die Eut- 

 wickelung unci Metamorphose der Polynoen." (Mutter's Archiv, 1841.) 



