1 68 AN INTRODUCTION TO ZOOLOGY 



coel.) communicates with the exterior by means of dorsal pores. 

 One of these is located in the mid-dorsal line at the anterior 

 edge of each somite from VIII or IX to the posterior end of the 

 body. 



General Internal Anatomy. If a specimen is cut open from 

 the anterior to the posterior end by an incision passing through 

 the body wall a trifle to one side of the mid-dorsal line, a general 

 view of the internal structures may be obtained (Fig. 87). As in 

 Ascaris, the body is essentially a double tube (Fig. 85), the body 

 wall constituting the outer, the straight alimentary canal, the 

 inner; between the two is a cavity, the ccdom. The external 

 segmentation corresponds to an internal division of the coelomic 

 cavity into compartments by means of partitions, called septa, 

 which lie beneath the grooves. These septa are absent in Ascaris. 

 The alimentary canal passes through the center of the body, and is 

 suspended in the coelom by the partitions. Septa are absent 

 between somites I and II, and incomplete between somites III 

 and IV, and XVII and XVIII. The walls of the coelom are lined 

 with an epithelium, termed the peritoneum. The ccelomic cavity 

 is filled with a colorless fluid which flows from one compartment 

 to another when the body of the worm contracts. In somites IX 

 to XVI are the reproductive organs (Fig. 93) ; running along the 

 upper surface of the alimentary canal is the dorsal blood vessel 

 (Fig. 85, dors. v.); and just beneath it lie the ventral blood vessels 

 (vent, v.) and nerve cord (n. co.). 



Body Wall. The strata of the body wall are as follows 

 (Fig. 85): (i) an outer noncellular covering, the cuticle (cut.); 

 (2) an epidermis (epid.) composed of two cellular layers, an outer 

 stratum consisting of gland, interstitial, and sense cells, and an 

 inner stratum of very small cells; (3) a layer of circular muscle 

 fibers (arc. mus.) running around the body, each fiber being long, 

 pointed at both ends, and longitudinally striated; (4) a thick 

 layer of muscle fibers running lengthwise of the body and appear- 

 ing in cross section to be arranged in featherlike groups (long. 

 mus.) ; (5) the innermost layer, the coelomic epithelium or perito- 



