PLANARIAN WORMS. 
393 
shapeless ovoid form of many Planarian species. They are in- 
stances of what is called 66 retrograde development;”* and we 
must not look at the modified shape and disposition of parts in 
the adult, so much as at the history and early condition of those 
parts, if we wish to see what are the true relations of the animal. 
The resemblance between Nudibranchs and Planarians is, when 
thus examined, found to be more apparent than real ; both live 
under the same conditions, beneath rocks and stones in the 
water, and exhibit identical modifications, but of very different 
types. They agree chiefly in their ramified intestine, flattened 
form, and nettle-cells in the skin. The relation to the Infusoria 
and to the Hairy-backed Animalcules (Chaetonoti) is a much 
truer one. Copious cilia, occasional stiff hairs, nematophores or 
trichocysts, pulsatiug water- vessels, and parenchymatous viscera 
are important characters met with in all three, while no corre- 
sponding differentiating characters exist. The young larval 
forms of some Turbellaria very closely resemble Infusoria and 
Chsetonoti too, whilst they approach the Annelid type as well. 
Turbellaria are either bisexual, or alternately male and female, 
or hermaphrodite. The Nemertians are believed all to be bi- 
sexual, whilst no Planarians are known to be permanently so. 
The female organs are an ovary, a vitellarium, a receptaculum 
seminis, and a uterus : the male organs are testes, vesiculse se- 
minales, a duct and penis, armed sometimes with a style. In 
the Nemertians, which are bisexual, only the essential organs 
are developed ; it is in the hermaphrodite species (Planarians) 
that here as elsewhere the most complex development of organs 
is found, of which we shall see more hereafter. 
The Turbellarians propagate either by eggs deposited and 
fertilised in the water, several eggs being often deposited in one 
mass of yelk (like what was observed by Dr. Carpenter in the 
Dog- Whelk), or by the growth of young from internal buds or 
pseud-ova, like the larvae of Cecidomya, or by transverse fission. 
Both Nemertians and Planarians exhibit these three methods. 
The young either develope directly, becoming similar to their 
parents at once ; or they exhibit a jointed ringed structure (like 
Annelids), sometimes too carrying bristles, as has been lately 
shown by Mr. Alexander Agassiz, both in Planarians and Ne- 
mertians, and then, as they grow older, lose their jointed appear- 
ance and setae ; or the egg-hatching results in a larva (Pilidium) 
which is totally unlike the parent, and from the body-wall of 
which a small worm-like animal grows and separates, leaving the 
bulk of the Pilidium to perish (fig. 2). This last case is very 
similar to that observed by Johannes Muller in certain star-fishes. 
As in the Echinoderms, so in the Turbellarians there appears to be 
The Cirripedes, Lernseans, and Linguatuli are similar instances. 
