514 ANNELIDA. 



skin is also often highly coloured, owing to the presence of 

 pigment, not only in the cutis, but to a certain extent also in the 

 epidermis. 



The clitellum consists of a glandular development of the skin of 

 the 9th, 10th, and llth segments (UOth, llth, and 12th). Amongst 

 the Gnatholtdellidae it is most developed in Neplielis; in Hirudo it 

 is inconspicuous. The genital openings are on the 10th and llth 

 segments* (Whitman, Oka) the male opening being in front of the 

 female (Fig. 415). 



According to the most recent statements the Hirudinea possess, without ex- 

 ception, 33 segments, some of which are however always reduced. A true 

 segment is determined by the presence of a ganglion, so that 33 ventral ganglia 

 can be made out. It must, however, be admitted that these ganglia are not 

 easy to see. The number is arrived at in this way : the suboesophageal ganglion 

 consists of 5 ganglia (Whitman, Oka) ; then follow 21 single free ganglia, and 

 then one composite one consisting of 7 fused ganglia. 



Apathy also makes 33 segments, but according to him the suboesophageal 

 ganglion consists of 6, and the last ganglion also of 6. His enumeration is as 

 follows. The body is divisible into six regions, viz. the region of (1) the head, 

 (2) the clitellum and preclitellum, (3) the stomach, (4) the intestine, (5) the anus, 

 and (6) the posterior sucker ; of these the anal region contains three segments, the 

 rest six each. The head region consists of six more or less reduced and shortened 

 segments, four or five of which may enter into the formation of the sucker. Segment 

 1 is always eyeless, and segment 6 possesses eyes only in the ten-eyed Gnathobdell- 

 idae. The mouth is on the ventral surface, and its anterior limit is at least two 

 segments from the front end of the body. The preclitellar segments are 7, 8, 9, 

 and the clitellar segments are 10, 11, 12. Of the 12 segments of the middle- 

 body, i.e., of the stomach and intestine, the first and last are usually reduced, 

 but the 2nd to the llth are complete, and each contains the number of rings 

 characteristic of the genus. The anal region contains three segments much 

 reduced ; the anus is in the dorsal middle line at most two segments in front of 

 the sucker. The post-anal segments are all incorporated in the sucker some of 

 them being invaginated into it ; they are six in number, as is shown by the 

 constitution of the last ganglion. 



The body-wall consists of a single layer of epidermis cells with a 

 cuticle; a cutis containing pigment cells and blood vessels, both of 

 "which intrude into the epidermis ; an outer circular and an inner 

 longitudinal layer of muscles. The muscular fibres are tubular, i.e., 

 they consist of an outer cortex of striated, presumably differentiated, 

 contractile substance, and an inner core of granular matter, in which 

 lies the nucleus. Unicellular glands, opening on the surface and 

 secreting a mucous fluid, are found in great numbers in the epidermis 



* Apathy makes the genital openings in segments 11 and 12, and the clitellum 

 to include 10, 11, and 12. Bourne agrees with the position in the text, if we 

 allow, as Oka does, that the suboesophageal ganglion consists of 5 fused ganglia. 



