THE PHYLUM ANNELIDA 



like opening in the last somite. The ventral side of the body is usually 

 marked by a slight flattening, and its color is often lighter than that of the 

 dorsal side. The somites — there may be 125 to 175 in a full-grown specimen 

 of Lumbncus terreslns — are clearly separated from each other by inter- 

 segmental grooves. A swollen, girdle-like area of glandular epithelium, the 

 clitellum, is usually located between somites 32 and 37; this secretes the egg 

 capsules at the time of oviposition. Other glandular areas are found on cer- 

 tain anterior segments. The openings of the ductus deferentes (vasa deferentia), 

 the male genital ducts, appear on the fifteenth somite as slit-like apertures 

 surrounded by conspicuous lips. The openings of the oviducts are much 

 smaller and are located on the fourteenth somite. In the grooves between 

 segments 9 and 10 and 10 and 11 are the openings of the two pairs of seminal 

 receptacles. The nephridiopores, openings of the nephridial tubules, which 

 occur on each somite except a few at the anterior and posterior ends, are 

 microscopic and variable in position. There are no structures resembling the 

 polychaete parapodia, but on each somite there are four pairs of minute setae. 

 These bristles, although small, are comparable with those found in the para- 

 podia of Nereis. Openings leading from the coelom, called the dorsal pores, 

 are located on the mid-dorsal line in the intersegmental grooves, except in the 

 first 10 or 12 somites of the anterior end. The coelomic fluid can thus be 

 exuded and spread over the surface of the body, presumably supplementing 

 the slimy secretions of epidermal gland cells. 



Structures and Functions Related to Metabolism. Internally, the struc- 

 ture of the earthworm resembles that of Nereis, although there are important 

 difTerences. The digestive tract, which extends throughout the length of the 

 body, is differentiated into a buccal cavity, a pharynx, an esophagus, a crop, a 

 gizzard, and a stomach-intestine which extends posteriorly to the anus (Fig. 

 14.7). The buccal cavity and pharynx, strongly muscular, function in apply- 

 ing the suction by which the animal ingests food. The esophagus is a passage- 

 way, provided with well-developed calciferous glands between somites 10 and 



Esophagus 



Pharynx 

 Ganglion 



Mouth 



Buccal 



cavity 



Nerve 

 cord 



Opening of 

 calciferous gland 



Septum 



Fig. 14.7. Sagittal section of the anterior end of an earthworm; semidiagrammatic. 

 from A. M. Marshall and C. H. Hurst, 1895, Practical ^oology.) 



(Modified 



405 



