ANNELIDA OR ANNULATA 115 



They noted that after conjugation, a new slime tube is formed 

 extending from the 7th to the 34th segment and that the eggs pass 

 back into the cocoon, before it leaves the region of formation at the 

 clitellum. Whether the sperms are squeezed out as the capsule 

 reaches the apertures of the seminal receptacles, or they are also 

 passed back to the capsule, the authors are not certain. 



Nervous System. — In the worms we have well-developed cerebral 

 and ventral ganglia, constituting the centralization stage in the 

 evolution of the nervous system of invertebrates. A bilobed brain 

 (paired cerebral ganglia) sends off two circumpharyngeal connectives, 

 which unite at the subpharyngeal ganglion. A ventral nerve cord 

 has a ganglion in each segment, with three pairs of lateral nerves. 

 Two pairs come off at the ganglia and one pair between the ganglia. 

 Afferent nerve fibers are sensory; efferent nerves are motor. Stough 

 (Jour. Comp. Neurol., vol. 40, no. 3, June, 1926) has shown that the 

 giant fibers, seen in cross sections of the earthworm nerve-cord, are 

 strictly segmental structures, and consist of a large number of 

 closely applied parallel axones. Epidermal sense organs, chiefly lo- 

 cated anteriorly and posteriorly, were discovered by Fanny Lang- 

 don when a college Junior. (Figure 50.) 



Behavior. — Earthworms react to the ordinary stimuli of light, 

 temperature, chemicals and electricity. They are very susceptible 

 to the contact stimuli produced by vibrations. It is reputed that 

 one way to drive earthworms from their burrows is to bore with a 

 sharp stake into adjacent soil. 



Several articles have appeared recently regarding the so-called 

 " singing " of earthworms. Apparently reliable reports have been 

 made of the peculiar noises, possibly due to the rasping of the setae 

 over stones or pebbles. Clark (Animals of Land and Sea) states 

 that the singing girls of Java sometimes swallow earthworms in 

 the hope that the tinkling sound will be " imparted to their voices." 



Class 3. Hirudinea — Leeches. — Hirudo medicinalis, the medic- 

 inal leech, has a deep olive hue, is velvety, two to three inches in 

 length, hermaphroditic and is found in Europe, America, Turkey 

 and Africa. Medicinal leeches live 15-20 years, and are adult at 

 5 years. They inhabit water; the female deposits 15-20 eggs in a 

 cocoon. These hatch in 3-4 weeks. 



External Anatomy. — Their external segmentation does not cor- 

 respond to the internal. There are usually 5 external grooves to 

 each segment. The medicinal leech has 2 suckers. 



