The Hirudinea of Illinois. 545 



EXPLANATION OF PLATES. 



The somites and ganglia are indicated by Roman numerals I to 

 XXVII; and the annuli of individual somites by letters (a, b, etc.) 

 which indicate the successive generations by which they multiply from 

 the triannulate type, the indices (1, 2, etc.) being their theoretical num- 

 ber in an antero-posterior series. 



With the exception of a few modifications which are explained in the 

 text the lettering is uniform for all of the figures. 



The metameric sensilla;: md, dorso-median; ill, dorso-lateral; dm, 

 dorso-marginal; sm, supra-marginal; sbm, sub-marginal; vl, ventro- 

 lateral; vm, ventro-median. 



The cutaneous papilhe: mp, median; mdp, dorso-median; dip, dorso- 

 lateral; dmp, dorso-marginal; snip, supra-marginal. 



General: a, anus; at, median atrium or penis sheath; atf, internal 

 elevation resulting from male pit (false atrium \ covered by a layer of 

 copulatory and prostate glands; ati, lumenal coat and sac of atrium; c, 

 clitellum; cgl, copulatory glands; egp, copulatory gland pores, $ of 

 male and 9 of female system; de, ductus ejaculatorius (variously modi- 

 fied and not always strictly homologous as indicated in the several 

 figures); ep, epididymis (remark under de applies to this also); g, 

 cutaneous glands; g XI to XVIII, ganglia of the ventral chain, num- 

 bered to agree with their somites; gp, (or pg), prostate glands and pros- 

 tate region of penial sheath; ga, glandula albuginea; np 1 to 17, 

 nephridial openings of pairs indicated by the numerals; od, oviduct; 

 ode, common oviduct; of, closed end of ovarian sac; os, ovisac (uterus); 

 ov, ovary or ovarian sac; ov' (in Fig. 27), position of ovary; p, prostate 

 cornua of atrium; pg (or gp), prostate glands and prostate region of 

 penial sheath; ss, sperm-sac; t 1, 2, etc., testes, numbered by pairs 

 from before backwards; va, vagina; vd, vas deferens; tf , male genital 

 orifice or its position; 9 > female genital orifice or its position. 



Unless otherwise stated all of the figures are made from specimens in 

 the Illinois collection and have been copied to scale as nearly as possible 

 after the originals, which were drawn upon camera tracings. Diagrams 

 are indicated. 



Plate XLII. 



Fig. 1. Placobdella parasitica. Dorsal view showing the metam- 

 erism and annulation of the twelve anterior somites; the cutaneous 

 papillae are not indicated. The color pattern is shown, the stippled parts 

 being the brown or olive background and the plain areas the yellow 

 spots and band. X 5. 



Fig. 2. Placobdella rugosa. Similar representation of the anterior 

 ten somites (except X a 3). The principal cutaneous papilla? are shown. 

 From a specimen taken near Philadelphia. X 5. 



