BRTTISH MARINE TTJRBELLARIA. 471 



Sub-family Plagiostomin^e. 



Plagiostomidse with a ventral and posterior genital 

 aperture. Mouth anterior. Gerraaria and vitel- 

 laria distinct. 



Genus 17.— Plagiostoma, O. Schmidt (28), 1852. 

 Without tentacles. 



31. Plagiostoma dioicum (MetschnikoiF, 37). PI. XL, fig. 11. 



Length -6 — -7 ram. Body cylindrical, tapering very 

 slightly in front of the posterior third. Colour yellow-brown, 

 lighter anteriorly and at the sides ; eye-pigment reddish- 

 brown. The epidermis contains a few small rods and nume- 

 rous highly refractive vesicles. Bohmig (57, p. 408) has seen 

 these in sections ; they are possibly excretory. Flagella are 

 present anteriorly and posteriorly. Mouth anterior, sub- 

 terminal. The pharynx is ellipsoidal, and lies altogether in 

 front of the brain. The intestine is extensive, and corre- 

 sponds generally to the form of the body. The brain is reni- 

 form, the slight concavity being directed anteriorly. A pair of 

 nerves from the anterior and also from the hinder angles are 

 conspicuous; three other pairs occur (Bohmig, 57, p. 410). 

 Two eyes are present, provided with lenses. The genital 

 aperture is ventral, and placed a short distance from the 

 hinder end. The testes are scattered in the parenchyma. A 

 muscular vesicula seminalis is placed close to the genital 

 pore. The female organs are of considerable interest, as no 

 one has yet found any trace of the yolk -glands. Since, how- 

 ever, it is known that these organs develop late, it is possible 

 that specimens presenting them in a mature condition have 

 not yet been seen. If, on the other hand, yolk-glands are 

 really not present at any stage, their absence would constitute 

 a feature in which this species resembles the genus Acmo- 

 stoma. The ovaries consist of a lateral row of clear spherical 

 cells extending from the brain to the hinder end, and sur- 

 rounded by refractive granules. I have not noticed the 



