1 6 PLATYHELMINTHES TURBELLARIA CHAP. 



" prostate " gland. The apex of the penis is eversible and not 

 merely protrusible, being turned inside out when evaginated. The 

 ovaries (Fig. 5, ov~) are numerous and somewhat spherical They 

 are dorsally placed, but when fully developed extend deeply 

 wherever they can find room to do so, and they not only furnish 

 the ova, but elaborate food-yolk in the ova, as there are no 

 special yolk-glands. The slender oviducts (od) open at several 

 points into the " uterus " (ut) (a misnomer, as no development 

 takes place within it), which encircles the pharynx, and opens by 

 a single duct into the vagina (va). Here the ova are probably 

 fertilised, and one by one invested by the shell-gland (5*7) with a 

 secretion which hardens and forms a resistant shell. They are 

 then laid in plate-like masses which are attached to stones or 

 shells. The development is a direct one, and the young Lepto- 

 plana, which hatches in about three weeks, has the outline of a 

 spherical triangle, and possesses most of the organs of the adult. 

 After leading a floating life for a few weeks it probably attains 

 maturity in about nine months. 



Classification, Habits, and Structure of the Polycladida. 



The Polyclads were so called by Lang on account of the numerous 

 primary branches of their intestine. They are free-living, purely 

 marine Platyhelminthes, possessing multiple ovaries, distinct male 

 and female genital pores (Digonopora), but no yolk-glands. The 

 eggs are small, and in many cases give rise to a distinct larval 

 form, known as " Muller's larva " (Fig. 12). The Polyclads, with 

 one exception, 1 fall into two sub-groups, Acotylea and Cotylea : 



Character. Acotylea. Cotylea. 



Sucker . . A sucker absent. 2 A sucker always present (Figs. 



8, D, s; 7, A, sc). 



Mouth . . In the middle, or behind the In the middle, or in front of the 

 middle, of the ventral sur- middle, of the ventral surface, 

 face. 



Pharynx . . More or less intricately folded. Rarely folded. Usually cylin- 

 drical or trumpet-shaped. 



Tentacles. . A pair of dorsal tentacles A pair of marginal tentacles (ex - 



usually present. cept in Anonymus). 



Development Usually direct. Larva when Muller's larva present. Metamor- 

 present, not a typical Miiller's phosis, however, extremely 

 larva. slight. 



1 Enantia spinifera Grff. Mittheil. d. Naturwiss. Vcrein. f. Steiermark, 1889. 

 The sucker of Leptoplana tremellaris probably does not correspond with that 

 of the Cotylea. 



