THE ANIMAL KINGDOM 415 



up of a number of similar (homoclynamous) segments or meta- 

 meres, united end to end. The body wall is covered on the out- 

 side by a cuticle, secreted by a single layer of underlying cells, the 

 epidermis (hypodermis). Beneath the epidermis are an outer 

 circular and an inner longitudinal layer of muscle. The inner 

 surface of the body wall encloses a coelomic space and is usually 

 lined by a peritoneum. As a rule, the coelomic cavity is sub- 

 divided by transverse septa (dissepimenta) into as many compart- 

 ments as there are complete body segments. Paired appendages, 

 when present, are not jointed. The subterminal mouth is over- 

 hung by a lobe, the prostomium, which represents the anterior end 

 of the animal. The prostomium may bear eyes, tentacles and 

 palps (tactile organs). The region about the mouth, the peri- 

 stomium, may bear tentacles, also known as cirri. The mouth 

 leads to a straight, differentiated digestive tract, sometimes 

 lobulated, terminating in an anus. A circulatory system, approxi- 

 mating a closed type, is present. The excretory system consists 

 of a segmentally arranged series of paired nephridial tubes, 

 described in the case of the earthworm in an earlier connection 

 (p. 146) . Likewise a description of the reproductive system of the 

 earthworm and the manner in which it functions has already been 

 given (p. 157). The central nervous system consists of a 

 circumpharyngeal ring, connecting ganglia above and below, the 

 latter joined with a ventral ganglionated nerve cord, extending 

 posteriorly the length of the body. Annelids are monoecious 

 (earthworms) and dioecious (marine worms). In the dioecious 

 annelids, the eggs, fertilized externally, develop into trochophore 

 larvae. The trochophore larva of the marine worm Polygordius 

 is unsegmented and is provided with an alimentary tract having a 

 right-angled bend in it. The exterior of the larva is at first 

 ciliated everywhere, but later the cilia become restricted to one or 

 more bands of epithelium, the ciliated bands, of which the preoral 

 band is shown in Fig. 241. The latter encircles the body, marking 

 off a prestomial area in front (above in the figure), which contains 

 an apical plate, beneath which is the rudiment of the supra- 

 esophageal ganglion. The larva also contains nephridial organs 

 opening to the outside. Most of the larva seems to represent the 

 head end {prosoma) of the fully developed worm. The segmented 

 trunk (metasoma) of the worm grows from the posterior edge of 

 the prosoma by the formation of segments one after the other. 



