6 OLIGOCHAETA 



not always appear to exist ; thus in Tubifex (Nasse) and Limnodrilus (Vejdovsky) no 

 Buch hollow sac is figured ; the setae and the solid mass of cells in which they are 

 imbedded reaching right up to the epidermis : in these cases however there seems to be 

 an iQvagination of the chitinous layer ; hence it is possible that a tube of epidermis is 

 also invaginated. Ceefontaine (1) has figured two sections out of a series through a seta 

 sac parallel to the surface of the body ; near to the external surface the seta is seen to 

 be surrounded by a tube whose walls are composed of about fifteen cells ; this region is 

 the tubular invagination of the epidermis ; further down the number is reduced to four 

 and then to two which are flattened and concave, closely embracing the setae, the 

 special lining of chitin present in the more superficial part of the tube being here 

 absent. 



In Anachaeta, as has been already mentioned, there are no setae at all; but the 

 recent existence of these structures appears to be shown by the presence of large 

 sacs depending from the epidermis on the dorsal side of the body, not the ventral, or 

 (Anachaeta eiseni) on both: these sacs are apparently the equivalents of the seta 

 sacs, and consist of a large cell of a glandular appearance with one or two nuclei 

 near to its free extremity. 



The form of the setae in the Oligochaeta is varied ; but they may be grouped into 

 two divisions, (i) long slender setae gradually diminishing in diameter towards the 

 pointed extremity, and (2) shorter setae of a curved form, something like an elongated 

 S with a thickening at about the middle; the shape of these setae has been aptly 

 compared to that of the mathematical sign /. The capilliform setae only occur in the 

 aquatic Oligochaeta, and not in all of them. 



The sigmoid setae vary very much in the details of their shape. The simplest form 

 is that which characterizes the vast majority of earthworms, the Phreoryctidae and many 

 Lumbriculidae. These setae end in a pointed extremity. In Onychochaeta the extremity 

 (fig. 2 a. 1 ) may be markedly hooked. In Pontoscolex among earthworms, in certain 

 Lumbriculidae, in all Tubificidae and Naidomorpha the sigmoid seta is cleft at the free 

 extremity (woodcut, fig. z b. 4). A further complication is seen in Tubifex and other 

 Tubificidae, where there are a few subsidiary prongs arising between the two main 

 prongs into which the extremity of the seta is cleft (woodcut, fig. a &. 6) ; this later 

 form of seta may be termed ' pectinate,' the simply cleft seta * uncinate.' A remark- 

 able variation characterizes Heterochaeta, and to a less extent Psammoryctes and 

 Spirosperma (woodcut, fig. 2 b. 3), where the widely divergent prongs are united by 

 a ribbed membrane. This form of seta has been termed * palmate.' Another form of 

 the uncinate seta characterizes certain Naidomorpha. The seta (woodcut, fig. a b. 7) is 

 straight instead of being curved, and has a cleft extremity. This form is termed 



