ANNELIDA. 55 



segment (peristomium). A small vertical oval opening, the anus, 

 is seen at the posterior end of the body. 



The eleventh, like all succeeding segments, has a minute 

 median dorsal pore, which communicates with the body-cavity, 

 and is placed in the groove separating the segment from the one 

 in front of it. Every segment, except the first three and the 

 last, possesses a pair of extremely small excretory apertures on its 

 ventral surface. 



The worm is hermaphrodite, and the openings of its repro- 

 ductive organs are found on certain of the anterior segments. 

 The dorsal and lateral regions of some of the segments between 

 29 and 36 inclusive (in L. herculeus, 32-37 inclusive) are thick- 

 ened into a band, the clitellum, which varies in size according to 

 the sexual condition. 



Eunning along each side of the body are two double rows of 

 minute, backwardly directed bristles, setce, which can be readily 

 felt by drawing a worm backwards between the fingers. One 

 row is lateral and placed where the pigment of the dorsal surface 

 shades off, the other ventral. Each segment, therefore, except 

 the first few and sometimes the last few, possesses eight setae. 



2. Skin. The body is covered by cuticular and epidermal 

 layers, internal to which come the muscles of the body-wall. 

 The cuticle is a very thin iridescent membrane, traversed by 

 numerous pores and of chitinous nature. It is secreted by the 

 underlying epidermis, which, except in the clitellum, is made up 

 of a single layer of nucleated columnar cells, many of which 

 are unicellular glands, of the type known as goblet-cells. These 

 are oval, and filled with liquid secreted by the cell-protoplasm. 

 The epidermis also lines deep narrow pouches (setigerous sacs), in 

 which are secreted the hard curved setae, which project at the 

 surface for only about one-fifth of their length. Each sac contains 

 one bristle, and when this falls out it is replaced by another 

 developed in the same sac. Muscular bands pass from the sacs 

 to the body-wall. The ventral setae of the genital and clitellar 

 regions are very slender. In the former region some of the 

 ventral setigerous sacs are glandular and much enlarged (capsul- 

 ogenous glands). 



The epidermis is much thickened and very glandular in the 

 clitellum, which also contains a network of blood-vessels. 



3. Digestive Organs (Fig. 18, A). The alimentary canal or gut 



