118 THE EARTHWORM 



hours; cells called neoblasts, arise on the posterior faces of certain 

 septa and migrate along the nerve cord to the posterior end where they 

 may be seen differentiating into mesodermal tissues. Record in a 

 table, or by means of drawings, such stages in this regeneration of 

 Tubifex as may be observed. 



Exercise 12. — Embryonic Stages. 



(c) Sexual union in earthworms results in an exchange of sperma- 

 tozoa, which are stored in the seminal receptacles. The egg-capsules, 

 or "eggs," arise from the clitellum as a secretion which hardens and 

 is passed along the body of the worm anteriorly; during this passage 

 several ova and many spermatozoa are discharged into the space be- 

 tween the hardened secretion and the body. Thus the zygotes become 

 enclosed in a capsule when the ends of the cylindrical secretion close 

 as it slips off the anterior end of the animal. Worms kept in a tub 

 of damp soil covered with a thin litter of leaves often lay egg-capsules ; 

 or capsules may be collected from the surface of ground, not covered 

 by vegetation, where worms are abundant. Examine capsules and the 

 embryonic stages that may be obtained by dissection from capsules. 

 The worms hatch as juveniles, several individuals having developed 

 within each capsule. Embryos and juveniles may be studied in water 

 in a watch glass and internal features observed as in Tubifex. Examine 

 whatever material is available and compare with textbook figures. 



OTHER ANNELIDS 



Exercise 1. — Representative Examples. 



(a) Examine any other annelids that may be available. There are 

 many more species in the ocean than elsewhere, although there are 

 many species of earthworms on land and many species of small anne- 

 lids in fresh water. Some of the latter may have been pointed out in 

 the study of Protozoa, as they are frequent inhabitants of such cul- 

 tures. Such forms as Tubifex, Dero, and yEolosoma can be used in the 

 living state without dissection to demonstrate the septa, nephridia, 

 nephrostomes, functioning of the circulatory system, peristalsis, ciliary 

 action in the intestine and the nephridia, ccelomic fluid, blood cells, 

 movements of the setae, and other activities. These small annelids are 

 available throughout the year in the mud at the margins or bottoms 

 of ponds and streams; some species can be kept alive almost indefi- 

 nitely in the laboratory. 



(b) Among the marine types, the clamworm, Nereis virens, is a 

 species that burrows in loose sand and swims freely in the water. 



