PLANARIAN WORMS. 
397 
simplest form, which form a body-wall of unlimited powers of 
movement. The large accumulations of cellular masses belong 
to the generative system, and may often obscure other parts. 
Quite in front is the pair of nervous ganglia (g) which send out 
their minute and invisible branches in all directions. In Opi- 
stomum there are no eyes to guide one in finding the ganglia, and 
hence the task is not so easy as in Mesostomum , Prostomum, 
and Vortex (figs. 7, 6, 8), in which the eyes lie just in front of 
the ganglia ; nor is there any expanse in front of the gan- 
glia — the “proganglionic region” is reduced to the smallest 
amount. Winding in and out among the contents of the body, 
sending out branches here and there, you will observe two 
indistinct tubes, running lengthwise, one on each side of the 
body ( nn ). These vessels are the main stems of the water- 
vascular system, and may be traced to two pores or openings 
placed far back on the ventral surface. Their internal surface 
is ciliated, but they are not contractile. 
The reproductive organs are the most curious and most im- 
portant points to make out in the straight-gutted Planarians ; 
and, indeed, in the Dendrocoels also they have been used as 
specific and generic characters. Although hermaphrodite, Opi- 
stomum , like other Rhabdoccels, has only one outlet for the 
ova and the seed. The Dendroccel Planarians are well divided 
into two groups, according as the generative opening is single or 
double (a male and female); and Elias Mecznikow* thinks that 
this may some day be done with the Rhabdoccels. The testes 
(fig. 5 t , f) in Opistomum form a large sac on each side of the 
body, opening into a middle seminal vesicle placed quite pos- 
teriorly ; leading from this is the very long coiled penis, which is 
covered with spines. In Prostomum (fig. 6pe ) there are in this 
position large and hard stylets, like the dart ” of the common 
snail, which some writers have considered as belonging to the 
male generative apparatus, and others have regarded as stinging 
organs. The ovaries ( ov ) are seen nearer the genital pore than the 
testes , and are smaller bodies ; they open into a cavity (ut) which 
has been called the uterus; into this also open two large 
glandular masses (vi) called <c vitellaria,” which in most animals 
form part of the ovary, but are here separated as distinct glands, 
as in many molluscs. They secrete the greater mass of the yelk, 
or food-yelk, as opposed to the essential formative part of the 
ovum, which is developed in the ovary ; just as in birds the 
food-yelk is secreted by the Graafian follicle. Besides these two 
glands, a cavity which receives the sperm during copulation ( rs ) 
opens into the uterus, and it is by this means that in the 
uterus the contact of the ova and spermatozoa is effected. The 
Wiegmann’s Archiv, 1865. 
