200 THE ANATOMY OF INV^ERTEBRATED ANIMALS. 



is hard to say what absolute charjicters separate these two 

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

 tella and Poli/op/U/iahnus, might be regarded as marine dioe- 

 cious JVaidce. But, in the higher Foli/chata^ each segment 

 of the body develops lateral piocesses — the parapodla^ or 

 rudimentary limbs, which are usually provided with abundant 

 strong setie; a distinct cephalic segment, the j^?r«s^o?>? /?/???, 

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

 tentacles; while those parapodia which lie in the vicinity of 

 the moutli may be specially modified in form and direction, 

 foreshadowing the jaws of the Art/wopoda. Ciliated, some- 

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

 of the segments may perform the office of external branc/ricc ; 

 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 Polynik will give a fair conception of a polychfe- 

 tous Annelid, in which the highest degree of complexity of 

 organization known in the group is attained : 



Polynoe squaniata is an elongated vermiform nnimal, 

 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. Tlius, then, the bod}^ of the Poh/noe is composed of 

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

 "segment," the prastonu'ion ("Kopf-lappcn," Grube), and 

 posteriorly by anotlier, the pyr/idiu?n, which may or may not 

 represent single somites. 



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

 51, C, P) 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, /) and the neuropodium 

 (A'), the one occupying the " haemal " or dorsal, the other the 

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



