1140 The American Naturalist. [December, 
ancestral form, and conclude that it is in the highest degree probable, 
inasmuch as we know several forms of animals which in their mature 
` condition come very near to the Trochophore. Particularly is this 
true for the Rotifers. The little spherical Rotifer discovered by Sem- 
per in the Philippine Islands illustrates most fully this law ; and it is to 
be marked that this is a typical Rotifer, and that at the same time many 
other Rotifers in spite of their changed outer form possess many true 
Trochophore characters. On the other hand, it has been affirmed that 
the Rotifers are sexually mature larve of higher forms, and this is not 
entirely impossible, but it must be remembered that there is no definite 
evidence for this hypothesis. It is further to be noted that the Tur- 
bellarians in their adult condition approach very near to the Protro- 
chula, except that in the adult form ciliated bands are wanting. The 
view that the inner organization of the Protrochula and Trochophora i 
repeat ancestral characters is made probable by the very similar rela- 
tions of the organs of the Platodes to the Rotifers. But even the 
outer arrangement of cilia of the Trochophore may in all probability 
be considered as an ancestral character, since it is found to someextent 
not only in the Rotifers but in other groups as well,—viz., in the 
Entoprocta and the Tentaculata as definite structures in the adult o 

organization. 
‘« We may thus formulate the following law: The Protrochula isa 4 
repetition of the Protrochozoon,—7.e., the common ancestral form of 4 
all Zygoneura. The Trochophora is the repetition of the Trochozoon, 
—i.e., the common ancestor of all the Zygoneura standing above the 
Platodes. ‘The organization of the Scolecida is referred directly back 
to the organs of the Protrochula and Trochophora. This applies to 
the nervous system, digestive tract, muscles, and the proto-nephridia, 
but not to the gonads, which appear primitively paired in the Scoleci- 
da, and have the structure of sac-gonads with peculiar excretory tubes. 
Concerning the development of these thege are few observations, but 
it is probable that the sac-gonads and ' gonad ducts (viz., egg- i 
tubes and sperm-tubes) are of mesodermal ı origin, and represent ccelo- 3 
matous formations 
‘« Kleinenberg had attempted to derivd the Trochophore fromthe * 
Medusa form, comparing the preoral ring-nerve of the Annelid larva 
with the ring-nerve of the Hydromeduse. This hypothesis is scarcely 
in accord with the rest of the organization. There is much better 
grounds for the belief that the Ctenophores stand very near to the 
Zygoneura. The sense-organ plate at the apical pole, the mesen- 
chymatous musculature and the ectodermal cesophagus appear to be 

