200 THE ANATOMY OF INVERTEBRATED ANIMALS. 



is hard to say what absolute characters separate these two 

 groups. The lowest forms of the Potychceta, such as Capi- 

 tella and Polyophthalmus, might be regarded as marine dioe- 

 cious Naidce. But, in the higher Polycha.ta^ each segment 

 of the body develops lateral processes the parapodia, cr 

 rudimentary limbs, which are usually provided with abundant 

 strong setae ; a distinct cephalic segment, the prcestomium, 

 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, some- 

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

 of the segments may perform the office of external branchiae ; 

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

 like processes, the so-called elytra. 



The following detailed description of a very common 

 species of Polynoe will give a fair conception of a polychae- 

 tous Annelid, in which the highest degree of complexity of 

 organization known in the group is attained : 



Polynoe squamata 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 / 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 prcestomium ( u 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 the body (Fig. 

 51, (7, D) be examined separately, 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, i) and the neuropodium 

 (&), the one occupying the " haemal " or dorsal, the other the 

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



