NO. 6 ANNELIDA, ONYCHOPHORA, AND ARTHROPODA SNODGRASS 37 



therefore, resemble the tentacular rudiments of the prostomium (A), 

 and later they grow out as tentaclelike processes. The bristle sacs 

 are formed as ectodermal cell masses between the cirri (B, chS), the 

 outer cells of which become myoblasts, while some of the inner cells 

 enlarge and produce the chaetae ; a lumen then appears in the cell mass, 

 and the latter becomes an open eversible sac from which the chaetae 

 protrude. Finally the cirri and the chaetal pouches are carried out- 

 ward on an outgrowth of the body wall that becomes the principal 

 part of the appendage. The mature parapodia of Lopadorhynchus 

 are not of typical form in that each consists of a single lobe (fig. i6 C) 

 with both chaetal sacs at its extremity. 



In some of the polychaetes, particularly in the Sedentaria, there 

 are two rows of podial organs on each side of the body (fig. i6F), 

 those of one series, the notopodia (dPd), being situated dorso- 

 laterally, those of the other, the neuropodia (vPd), ventrolaterally. 

 Each organ includes a cirrus (Cir) and a chaetal sac (chS), and 

 is innervated separately from the corresponding podial ganglion 

 {PdGng). In the Oligochaeta the podial organs are represented only 

 by the chaetae, which usually are arranged in two separated rows on 

 each side of the body. It is possible, therefore, that the usual two- 

 branched parapodium of the Polychaeta (fig. 15 B) has been formed 

 by the union of a notopodium and a neuropodium. Furthermore, the 

 double composition of each notopodium and neuropodium suggests 

 that the primitive polychaetes had dorsolateral and ventrolateral rows 

 of cirri, and between them on each side two series of chaetal sacs. 

 On the peristomial segments of adult polychaetes generally only the 

 cirri are present (fig. 14 A, Ctrl, Cirll), but on the rest of the body 

 segments the chaetae-bearing lobes are usually the more important 

 podial elements. 



The musculature of a parapodium is somewhat complex : it includes 

 extrinsic muscles that move the appendage as a whole, and intrinsic 

 muscles concerned principally with the movement of the chaetae. In 

 Nereis virens there are four extrinsic muscles for each parapodium, 

 two dorsal (fig. 15 D, 7, S), and two ventral (p, 10). The dorsal 

 muscles arise anteriorly and posteriorly on the body wall, but cross 

 each other obliquely to opposite margins of the parapodial base. The 

 ventral muscles take their origins on the median infold of the ventral 

 wall of the body segment (A), and extend laterally and dorsally, 

 above the ventral longitudinal body muscles (4), to the anterior and 

 posterior margins of the base of the parapodium. If the dorsal and 

 ventral muscles inserted anteriorly act in opposition to those inserted 

 posteriorly, the parapodium is moved anteriorly and posteriorly on 



