THE STRUCTURE OF THE EARTHWORM. 195 



transverse grooves which divide its body into iiumerous nar- 

 row rings or segments/ The most anterior segment is small 

 and conical, and presents, on its under surface, a depression 

 which is the oral aperture. The anus is at the opposite end 

 of the body. Behind the mouth, the successive segments rap- 

 idly attain their average size ; but, in a full-grown worm, a 

 part of the body, into which more or fewer of the segments 

 between the twenty-fourth and thirty-sixth inclusively (29 ?- 

 36, X. terrestris ; 24-29 ?, L. ruhellm ; 26-32, L. communis) 

 enter, is swollen, of a- different color from the rest, provided 

 with abundant cutaneous glands, and receives the name of 

 cingulum or clitellum. 



In the dorsal median line there is a series of small aper- 

 tures or pores, one for each segment except the most an- 

 terior, which lead into tlie perivisceral cavity ; while upon 

 the ventral surface of the anterior part of the body the eight 

 apertures of the organs of generation are situated. Of these, 

 four, situated two on each side, between the ninth and tenth, 

 and the tenth and eleventh segments, are the openings of the 

 receptacula semlnis. The openings of the two oviducts are 

 on the fourteenth segment ; those of the two vasa deferentia 

 on the fifteenth. Besides these, all the segments, except 

 some of the most anterior, exhibit a pair of minute openings 

 appertaining to the segmental organs ; and they are further 

 perforated by the four longitudinal double rows of set^, 

 which project slightly beyond the surface of the integument, 

 and offer a certain resistance when the worm is drawn from 

 tail to head through the fingers. 



The body is invested in a thin and transparent but dense 

 ciitlcula^ perforated by excessively minute vertical canals. 

 Within this lies a thicker layer, consisting of a reticulated 

 nucleated protoplasm, the meshes of which are filled with 

 a transparent gelatinous substance. This layer probably rep- 

 resents both the dermis and epidermis^ and has been termed 

 the hypodermls. Internal to it lies a thick layer of circular 

 muscular bands, in the interstices of which pigment-granules 

 occur ; and, still more internallj^, is a much thicker coat of 

 muscular fibres, which are disposed longitudinally. 



The cavity circumscribed by this longitudinally fibrous 

 muscular layer is lined by a kind of connective tissue. Cor- 

 responding with the divisions between every pair of segments 



1 The question how far all these segments represent somites may be left 

 open. The history of the development of the Earthworm is in favor of their 

 being true somites. 



