BY WILLIAM A. IIASWELL. 429 



One section of the Vorticida, including Auoplodiumf and 

 Graffilla,\ are parasitic; but the new form is not in anyway 

 related to these, which have the brain and the pharynx greatly 

 reduced. 



The following appear to be the leading characteristic features 

 of the new form, fur which I propose the name Didymorchis 

 paranephropis : — Pharynx a " pharynx doliiformis." Excretory 

 system opening on the ventral surface by two apertures situated 

 not far from the mouth and pharynx. A single reproductive 

 aperture. A single compact ovary and two elongated vitelline 

 glands. No bursa copulatrix. A pair of compact testes situated 

 posteriorly in close contact with one another. A complex chiti- 

 nous penis consisting of a tubular basal part and a distal system 

 of spines. 



EXPLANATION OF PLATES XX. -XXI. 

 Plate xx. 



General view of the external features and internal organisation of Didy- 

 morchis from the dorsal aspect, (x 250). 



a.v., anterior main vessel of excretory system, exc, opening of excretory 

 system, g.o., genital opening, int., intestine, oot., ootype (uterus), ov., 

 ovary, p., penis, ph., pharynx, p. v., posterior main vessel of excretory 

 system, r.v., vitelline receptacle, t.t., testes, vit., vitelline glands. 



Plate xxi. 



Fig. 1. — Ventral view of the reproductive apparatus, gr.d.. granule ducts. 



r.v.d., right vas deferens. te., te., testes. Other letters as in 



Plate xx. 

 Fig. 2. — Outline of female reproductive organs of another specimen, dorsal 



aspect, J opening of vagina. sp., mass of sperms in oviduct. 



Other letters as in Plate xx. 

 Fig. 3.— Dorsal view of chitinous parts of penis. 



t A. Schneider, " Ueber einige Parasiten der Holothuria tubulosa i. 

 Anoplodium parasita." Midler's Archiv, 1858. Von Graff, torn. cit. p. 376. 



I H. von Ihering, " Graffilla muricicola, eine parasitische Rhabdocoele." 

 Zeitschr. f. wiss. Zoologie, Bd. xxxiv. 1880. 



