Our Fresh-Water Planarice. 451 



naturally, as in the Vorticellce and other Infusoria. Miiller 

 noticed it in P. ciliata, Duges in Derostoma leucopta, Otto Fabri- 

 cius in P. vulgaris. Draparnaud and Duges saw it several 

 times in P. tentaculata. I have frequently experimented 

 myself in this way, and have seen these creatures reproduce 

 lost halves, or segments ; but on chopping them into eight or 

 ten parts, I have always found they died, doubtless from some 

 causes not favourable to reproduction. An individual divided 

 in a longitudinal fissure at the head will often exhibit the 

 phenomena of two heads. Planarice propagate by mutual 

 contact, as in other animals ; they are androgynous ; and no 

 doubt the presence of two individuals is necessary for the pro- 

 creative act. Duges has witnessed the copulation in P. torva, 

 and recently I have noticed it in the same species. Accord- 

 ing to the researches of Duges, " the male organ consists 

 of two parts, one of which is free, smooth, semi-transparent, 

 contractile, and always divided into two portions by a 

 circular constriction ; it is traversed by a central canal, 

 susceptible of being dilated into a vesicle, and is open at its 

 free extremity, which is turned backwards ; the second division 

 is thicker, more opaque, vesicular, adherent to the contiguous 

 parenchyma, and receives two flexuous spermatic canals. The 

 free portion of this organ is contained within a cylindrical 

 muscular sheath, which is adherent to the circumference of the 

 base of the intermittent organ, and serves to protrude it 

 externally. This sheath communicates with the terminal sac of 

 the female apparatus near its outlet by a projecting orifice. 

 The oviduct opens into the posterior part of the terminal sac ; 

 it is a narrow tube which passes directly backwards, and 

 dividing into two equal branches, again subdivides, and ramifies 

 amongst the branches of the dendritic digestive organ. Besides 

 the ovary, there are two accessory vesicles, communicating 

 together by a narrow duct, and opening into the terminal 

 generative sac." The' Planarice lay round or ovoid eggs, with 

 a horny covering, containing three or four young ones. These 

 cases are of a reddish colour, and may be found in numbers 

 deposited singly on the leaves of aquatic plants, especially 

 within the stems of Sparganium. I believe the young ones 

 issue from the capsules in eight or nine days after they have 

 been laid ; the young ones are minute and drab-coloured, about 

 a line long ; they exactly resemble the parents, both in form 

 and manuer of life ; they grow rapidly where the water is fresh 

 and the food plentiful.* 



* Other modes of propagation have been observed to take place in some of 

 the Turlellaria, as by internal budding, or the young are at first larva; unlike 

 their parents. Professor Agas3iz once thought that certain infusoria, as Parame- 

 cium and Kolpoda, were Planarian larvae ; but the researches of Balbiani have 



