GEPHYREA : HIRUDINEA. 637 



Lampert, Z. W. Z. xxxix. 1883; Sluiter, Tijdschrift Nederl. Indie, 43, 1883; de- 

 velopment of, Conn, Studies Biol. Lab. Johns Hopkins Univ. iii. pt. 7, 1886. 

 Bonellia, Rietsch (supra}-, de Lacaze Duthiers, A. Sc. N. (4), x. 1858; development 

 of, and male, Spengel, Mitth. Zool. Stat. Naples, i. 1879 ;./% of male, Selenka, 

 ' Gephyrea,' Challenger Reports, xiii. 1885, PI. ii., figs. 7-10. Bonellein, Krukenberg, 

 Vergleich. Physiol. Studien (2), ii. 1882. Hamingia, Ray Lankester, A. N. H. (5), 

 xi. 1883. 



CLASS HIRUDINEA. 

 (Discophora). 



Multisegmental Vermes, in which the original somites are masked by 

 the development of secondary annuli. The posterior somites are always fused 

 to form a terminal sucker. The original coelome is obliterated for the most 

 part by a growth of connective tissue, and is represented by a series of sinuses 

 and channels with which the true blood-system communicates. The mouth is 

 anterior and ventral, and the anterior extremity of the body in which it lies 

 is. often marked off as a more or less efficient sucker. The anus is dorsal and 

 anterior to the posterior sucker. Hermaphrodite. The genital openings are 

 median and ventral ; the male in front of the female. For the most part 

 aquatic, parasitic, and blood-sucking. 



The ventral surface of the body is flattened, the dorsal convex. The 

 animal creeps by alternately attaching and detaching the anterior and pos- 

 terior extremities or suckers, and swims by undulations of the body. The 

 body is segmented. The Hirudinidae possess twenty-six somites, and 

 these somites in them as well as in all other Leeches are annulated. A 

 variable number of annuli, or rings, belong to a somite, e.g. in Hirudo five, 

 Pontobdella four, Branchellion three. In the cephalic and posterior regions 

 of the body, the number in the somite is less, and may most anteriorly be 

 reduced to a single ring. A first annulus in the middle region of the 

 body contains a nerve ganglion and a nephridial runnel, and it may be 

 marked externally by peculiar arrangements of colour, of integumental 

 papillae, or by the presence of segmental sensory papillae. The anterior 

 extremity of the body is more or less expanded, and its annulation lost in 

 the Leeches with a pharyngeal proboscis (Rhynchobdellidae\ but whether 

 expanded or not, it always acts as a sucker. The two or three somites 

 which contain the genital openings constitute the clitellum, and at the re- 

 productive season secrete on their surface the cocoon in which the ova are 

 contained. These genital somites are the ninth, tenth, and eleventh in 

 Hirudo (Whitman) ; the sixth and seventh post-oral of Pontobdella (A. G. 

 Bourne). The marine Branchellion Torpedinis has a pair of lateral folia- 

 ceous expansions to each annulus of the middle and posterior regions of 



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