A few _Mesostoma_ (_M. ehrenbergii_, _M. productum_, _M. lingua_) produce two kinds of eggs—thin- and thick-shelled. The latter are laid throughout the summer, and lie dormant through winter. The young which hatch in spring out of these "winter" eggs develop rapidly, and when only 7 to 8 mm. long (_i.e._ one-third the size of the parent) already possess functional genital organs; the penis, however, is rudimentary, and incapable of being used for copulation. Hence it is probable that this stunted progeny self-fertilise their thin-shelled or "summer" eggs. After the formation of these eggs the same parent is said (Schneider[69]) to produce thick-shelled or winter eggs, but however that may be, the first young which hatch from the thin-shelled ova are produced in great numbers at a time (April to May) when food is abundant. These grow rapidly to the full size, and then having attained maturity, cross-fertilise one another's ova, which become encased in a thick brown shell; and it is these numerous "winter" eggs that lie dormant throughout the autumn and winter. Many _Mesostoma_, and practically all other Rhabdocoela, however, produce only thick-shelled eggs, and in all cases it is probable that to these many species owe their wide distribution, the exact range of which is, however, unknown, as is also the means of dispersal. {49}CLASSIFICATION OF RHABDOCOELIDA. ACOELA. Family. Genus and British species. PROPORIDAE _Proporus venenosus_ O. Sch. Plymouth. _Monoporus rubropunctatus_ O. Sch. Plymouth. _Haplodiscus._ APHANOSTOMATIDAE _Aphanostoma diversicolor_ Oe. Common. _A. elegans_ Jen. Plymouth. _Convoluta saliens_ Grff. Plymouth, Millport. _C. paradoxa_ Oe. (Fig. 19, B). Common. _C. flavibacillum_ Jen. Plymouth, Port Erin, Millport. _Amphicoerus._ _Polychoerus._ RHABDOCOELA. MACROSTOMATIDAE _Mecynostoma._ _Macrostoma hystrix_ Oe. Stagnant water. _Omalostoma._ MICROSTOMATIDAE _Microstoma lineare_ Oe. Fresh water. _M. groelandicum_ Lev. Plymouth, among _Ulva_. _Stenostoma_ (_Catenula_) _lemnae_ Dug. Near Cork. _S. leucops_ O. Sch. Common in fresh water. _Alaurina claparedii_ Grff. Skye. PRORHYNCHIDAE _Prorhynchus stagnalis_ M. Sch. In Devonshire rivers. _Promesostoma marmoratum_ M. Sch. Common. _P. ovoideum_ O. Sch., _P. agile_ Lev. Plymouth. _P. solea_ O. Sch. Plymouth, Port Erin. _P. lenticulatum_ O. Sch. Port Erin.