THE ANATOMY. SETAE 



Fio 



they are in this family nearly always unpaired and median. The pores of the 

 spermathecae are not always so visible ; they are however for the most part in the 

 Eudrilidae, where the pore is single and median. The accompanying plate (PL I) 

 shows how great the external differences may be between earthworms, though in 

 general shape they are so much alike. 



§ 4. Setae. In all Oligochaeta, with the single exception of Anachaeta (woodcut, 

 fig. i), there are chitinous rods formed by epidermic cells and arranged in a certain 

 definite plan, which are most usually 

 termed ' setae ' ; these setae are partly 

 buried in the thickness of the body-wall, 

 and are prolonged into the body-cavity ; 

 the free extremity projects for a varying 

 distance beyond the epidermis ; the setae 

 are the organs of locomotion of the animals ; 

 they are furnished with special muscles 

 which enable them to be retracted or pro- 

 tracted and pulled forwards or backwards ; 

 progression is effected by their movements. 



It used to be believed that the setae 

 were structures of mesoblastic origin; but 

 it is now, through the researches of 

 Vejdovsky and others, so firmly established 

 that they are epidermic that it seems to be 

 of no pai*ticular use to go into the history 

 of the older and erroneous view. The setae 

 are implanted in sacs which are diverticula 

 of the epidermis ; at the margin of these sacs the chitinous cuticle is invaginated and 

 forms a lining for the seta sac down to a certain depth ; beyond this the seta is imbedded 

 in a mass of cells, each one of which can produce a new seta to replace the original one. 

 The invaginated part of the epidermis consists of a row of low cells lined by the cuticle 

 already mentioned, and terminating in the mass of cells without cell boundaries in 

 which the seta is firmly imbedded ; often, as Vejdovsky has figured for Rhynchelmis, 

 the sac is reinforced by a second sac lying near to it, also multicellular and containing 

 a seta in course of development. The setae first appear as small cones of chitinous 

 substance ; the apex of the seta, its free extremity, is first developed ; it then gradually 

 grows in length. A single sac (which it must be remembered is multicellular) often 

 contains, as in the Tubificidae, a large number of setae. The invaginated epidermis does 



ANACHAETA BOHEMICA. 

 (After Michaelsen.) 



t. Grland cells = dorsal setae of other forms. 2. Clitellar 

 epidermis. 3. Lateral line. 4. Openings of sperm-ducts. 

 5. Nerve-cord. 6. Chloragogen-cells. 7. Epithelium of 

 gut. 8. Ventral blood-vessel. 9. Blood-sinus of alimentary 

 canaL 10. Longitudinal muscles. 



