I ANATOMY OF RHABDOCOELIDA 47 



genera (Mesostoma, Vortex). But Macrorhynchus helgolandicus, 

 though minute (1*5-2 mm. long), has a more complex structure 1 

 than any other species of the specialised marine genus to which 

 it belongs, and is a remarkable instance of great complexity 

 being associated with small size. 



Reproduction. The Rhabdocoelida present the greatest 

 diversity in the development of the reproductive system. The 

 Acoela and Alloeocoela have the simplest arrangement. Scattered 

 testes, often without a distinct membrane, form the spermatozoa, 

 which in most cases wander into parenchymatous spaces, but 

 in Monoporus rubropunctatus and Bothrioplana, into distinct vasa 

 deferentia. In both groups a protrusible penis opens inde- 

 pendently to the exterior, and may be simply muscular or provided 

 with a chitinous armature. Two ovaries are present, and the 

 oviducts, if distinct, are continuations of the ovarian membrane. 

 In most forms a " bursa seminalis," which receives the sperma- 

 tozoa of another individual, is appended to the female genital 

 canal. In many of the Alloeocoela, however, a portion of the 

 ovary is sterile, and its cells, forming a yolk-gland, feed the 

 fertile portion, the whole structure being then spoken of as a 

 germ- yolk-gland. In many others (Monotidae) this sterile part 

 has become an independent yolk-gland, which communicates by 

 yolk-ducts with the oviducts. The Acoela form no egg-case, the 

 body of the parent becoming a bag for the ova, which elaborate 

 their own food -yolk. The Alloeocoela lay hard -shelled eggs, 

 which are produced in Bothrioplana and Automolos by the activity 

 and interaction of reproductive organs, resembling closely those 

 of certain Triclads. 2 



The Ehabdocoela exhibit every stage in the development of a 

 complex reproductive system, from the simple ovaries and testes 

 of a Microstoma or Macrostoma, to the intricate system of ducts 

 and glands of a Macrorliynclius (Proboscidae), in which there is 

 still much to be made out. The complications of the copulatory 

 organs chiefly arise from the way in which the spermatozoa are 

 brought into contact with a nutritive prostatic fluid, or axe 

 formed into spermatophores ; and also from the penial armature, 



1 See von Graffs Monographic, pi. ix. ; and Jensen, Turbcllaria ad Liiora 

 Xorvcgiac, Bergen, 1878, pi. iv. 



2 For the reproductive organs of Rhabdocoelida consult von GratF, Mono- 

 graphic, "Die Acoela" ; and Bbhniig, Zcitschr. f. iciss. Zool. Bd. li. 1891, p. 167. 



