BRITISH MARINE TURBELLARIA. 193 



at the same time discharging some of its contents. This oral pair of 

 poison-organs is constantly present in sexually mature individuals. 

 When the male products ripen (this species is a distinctly protandrous 

 hermaphrodite) two other pairs (according to v. Graff) arise close to 

 the male genital pore. They differ from the oral pair in three points — 

 their time of appearance, their variability (the hinder pair may be 

 absent), and their disappearance after the shedding of the male 

 products (see v. Graff, 44, p. 61). The female genital pore lies slightly 

 in front of the centre, the male pore halfway between it and the 

 hinder end. The testes are dorsal in position. The penis is a narrow, 

 cylindrical, muscular tube opening into the short genital atrium, 

 surrounded by radiating accessory glands. The spermatozoa are long 

 (•22 mm.), and consist of a finely granular central portion and hyaline 

 borders, absent, however, on the distinct " tail." The ova are placed 

 ventrally, and when mature are ovoid, "07 mm. diameter, containing 

 much yellow food-yolk, and surrounded by a delicate membrane. 

 Thirty to forty fertilised eggs may be present at one time in a single 

 example. A spermotheca is present. Its neck is produced forwards 

 into a funnel-shaped expansion opening into the female atrium and 

 backwards into the swollen basal portion containing spermatozoa. 

 Round the neck chitinous plates are arranged one over another, the 

 margins of which are thin and colourless, forming the " mouth-piece." 

 Young specimens of Convoluta paradoxa differ from the adults, to 

 which the foregoing description applies, chiefly in the absence of 

 reflexed marginal lappets, and the small number (five to seven) of 

 symbiotic algse present. 



Habitat. — A littoral species, occurring among seaweeds in tide-pools 

 all round our coast. Berwick Bay (Johnston) ; Firth of Forth 

 (Dalyell) ; among Ceramiae, Weymouth (Gosse) ; Skye (Claparede) ; 

 Guernsey (Lankester) ; St. Andrews (Mcintosh) ; Millport (v. Graff) ; 

 Plymouth, Port Erin, Isle of Man (F. W. G.). 



Distribution. — Mediterranean, Adriatic and Black Seas, North Sea, 

 North Atlantic, coast of Denmark. 



7. Convoluta flavibacillum, Jensen. 



Length 2 — 3 mm. Body stout, oval, pointed behind, dorsal surface 

 convex, ventral surface flat, The margins are produced into thin 



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