200 THE ANATOMY OF INVERTEBRATED ANIMALS. 



is hard to say what absolute characters separate these two 

 groups. The lowest forms of the Polycha^ta^ such as Capi- 

 tella and Polyophthahnus^ might be regarded as marine dioe- 

 cious KaidcB. But, in the higher Folychoeta^ each segment 

 of the body develops lateral processes — the parapodia^ or 

 rudimentary limbs, which are usually provided with abundant 

 strong set« ; a distinct cephalic segment, the p>rcestomii(m^ 

 appears in front of and above the mouth, and bears eyes and 

 tentacles ; while those parapodia which lie in the vicinity of 

 the mouth may be specially modified in form and direction, 

 foreshadowing the jaws of the Arthropoda. Ciliated, som,e- 

 times plumose, processes of the dorsal walls of more or fewer 

 of the segments may perform the office of external hrancldoe j 

 and, occasionally, the dorsal surface gives rise to flat shield- 

 like processes, the so-called elytra. 



The following detailed description of a very com.mon 

 species of Polynoe will give a fair conception of a pol3'ch8e- 

 tous Annelid, in which the highest degree of comj^lexity of 

 organization known in the group is attained : 



Polynoe sqiiamata is an elongated vermiform animal, 

 about an inch long, the body of which is divided into a suc- 

 cession of portions, for the most part similar and equivalent 

 to one another, but presenting peculiar modifications at the 

 anterior and posterior extremities. Each such portion is 

 properly termed a somite j while the term "segment" may 

 be retained to indicate generally a portion of the body, with- 

 out implying its precise equivalency to one somite or to 

 many. Thus, then, the body of the Polynoe is composed of 

 a series of twenty-six "somites," terminated anteriorly by a 

 "segment," the prcestoynium ("Kopf-lappen," Grube), and 

 posteriorly by another, the pygidium^ which may or may not 

 represent single somites. 



If one of the somites from the middle of thp body (Fig. 

 51, <7, Z>) be examined separatel}^ it will be found to be 

 transversely elongated, so as to be about three times as broad 

 as it is long, and to be slightly convex above and below, 

 presenting a deep, median, longitudinal groove inferiorly. 

 Laterally the somite is produced into two thick processes, 

 the '•'' parapodia, ^"^ 



Each parapodium divides at its extremity into two por- 

 tions, a superior and an inferior, which may be denominated 

 respectively the notopodium (Fig. 51, ^) and the neuropodiiim 

 (Jc)^ the one occupying the " ha?mal " or dorsal, the other the 

 "neural" or ventral aspect. The latter is, in this species 



