NO. 6 ANNELIDA, ONYCHOPHORA, AND ARTHROPODA SNODGRASS 35 



can produce only contraction or lateral undulatory movements of the 

 body; the circular muscles are constrictors producing peristaltic waves 

 of body compression, and longitudinal extension of the body by the 

 creation of internal pressure. The arthropod type of body mechanism, 

 involving intersegmental movement of integumental plates, can be 

 derived from the intrasegmental annelid mechanism only by the 

 establishment of new intersegmental divisions. 



The polychaete somatic musculature is well developed in the 

 Nereidae, of which Nereis virens may be taken as an example (fig. 

 15). The outermost layers of body wall muscles consist of fine cir- 

 cular fibers closely adherent to the integument (A, D, /). Internal 

 to these there may be bands of oblique fibers (D, 2) crossing each 

 other in opposite directions. The largest of the somatic muscles, 

 however, are four thick bundles of longitudinal fibers (A, j, 4) lying 

 internal to the others, two dorsal and two ventral, the fibers of which 

 are attached on deeply inflected intersegmented folds of the integu- 

 ment (D, isf). The longitudinal muscles of the terrestrial oligochaetes 

 are continuous in a thick layer around the entire circumference of 

 each somite, except where they are interrupted by the intrusion of 

 the four chaetal sacs. Besides the muscles of the body wall there is 

 in Nereis a double series of paired, obliquely transverse ventral 

 muscles, one pair anterior and the other posterior in each segment 

 (D, 5^ 6), which extend outward and upward from the median ventral 

 fold of the body wall (A) to the lateral intersegmental folds between 

 the parapodial bases. The intersegmental folds give attachment also 

 to the transverse or radial muscles of the intercoelomic dissepiments. 

 Most of the other muscles of the body pertain to the chaetal sacs and 

 the parapbdia, and will be described in connection with the parapodia. 



A typical polychaete parapodium is a lateral outgrowth of the body 

 wall (fig. 15 A, Papd), flattened antero-posteriorly, and usually 

 divided into a dorsal lobe and a ventral lobe, which again may be 

 subdivided into secondary lobules. Each major lobe bears distally a 

 fan-shaped group of long chaetae (B, C/z), and on its base a slender 

 cirrus {dCir, vCir). The chaetae arise from the inner walls of chaetal 

 sacs (C, chS), from each of which a long rod, the acicula (Acic), 

 extends inward to give attachment to protractor and retractor muscles. 



The larval rudiments of the parapodia represent the cirri and the 

 chaetal sacs, and are differentiated as cellular bodies within the ecto- 

 derm. The rudiments of the cirri, as described by Kleinenberg 

 (1886) and by Meyer (1901) in the larva of Lopadorhynchus (fig. 

 16 B, dcR, vcR), consist each of an outer layer of myoblasts (m) 

 and an inner core of sensory nerve cells (n). The cirri in their origin, 



