x PHYLUM ANNULATA 465 



of ganglia. The sexes are united in Protodrilus, ovaries occurring in all the first 

 seven segments, and testes in some of those immediately following. In Poly- 

 gordiusthe sexes are separate ; the ovaries or testes (Fig. 367, spy] are developed 

 in the posterior segments. There are no special reproductive ducts. 



The larva of Polygordius is a typical trochosphere (Fig. 368), and its meta- 

 morphosis into the adult worm (Fig. 369) takes places as in the Polychseta in all 

 essential respects. 



CLASS IV. HIRUDINEA. 



1. EXAMPLE OF THE CLASS The Medicinal Leech (Hirudo mcdici- 



nalis and H. quinquestriata). 



The medicinal Leech is found in ponds, swamps, and slowly 

 flowing streams in many parts of the world. If. medicinalis is the 

 common British species : H. quinquestriata is an allied Australian 

 form. 



External Characters. It is a vermiform animal, some 

 6-10 cm. (2-3 inches) in length, but is capable of contracting and 

 elongating itself so as to produce great alterations in form and 

 proportion. It moves by " looping " movements, and is also a 

 good swimmer. The body (Fig. 370) is depressed or flattened dorso- 

 ventrally, the dorsal surface convex, the ventral flattened. The 

 anterior end presents a ventrally directed cup-like hollow, the 

 anterior sucker (a. s.), in the middle of which is a small aperture, 

 the mouth (mth.). The hinder end bears a disc-like posterior sucker 

 (p. s.), also directed downwards, and at its junction with the trunk, 

 on the dorsal surface, is the very small median anus (a.). The 

 animal is brightly coloured, the dorsal surface in If. medicinalis 

 being longitudinally banded with alternate stripes of greenish grey 

 and rusty red, the ventral surface greenish yellow, spotted with 

 black : in If. quinquestriata the whole under-surface is rust- 

 coloured. 



The whole body is encircled by close-set transverse grooves, 

 dividing it into annuli. These, like the annuli of some Earthworms, 

 are more numerous than the true segments or metamere.s, the 

 study of the internal organs showing that, except at the two extre- 

 mities, each segment contains five annuli. There are also external 

 characters by which the actual segmentation is plainly indicated. 

 The rust-coloured streaks on the back ofH. medicinalis are spotted 

 with black, and at every fifth anrmlus the spots are larger than on 

 the intervening rings : the annuli thus marked are the fifth or last 

 of their respective segments. Moreover, the same rings bear on 

 the ventral surface minute paired apertures, the ncphridioporcs or 

 excretory apertures (n.p.\-Ul): of these there are altogether 

 seventeen pairs, marking the fifth rings of the sixth to the twenty- 

 second segments. 



VOL. I H H 



