526 THE AMERICAN NATURALIST. [Vor. XXXIV. 
contraction of the mantle musculature the water is expelled through 
the posterior opening, thus driving the body of the animal forwards. 
By a succession of such jets the animal may swim some feet before 
settling. P. 
Ear Bonés.— The ear bones of vertebrates have undergone a 
careful comparative examination at the hands of J. S. Kingsley.’ 
In urodeles and ccecilians, where no tympanum exists, a stapes, 
which develops independently of the otic capsule, is the only ele- 
ment present. In the anura the space between the otic capsule and 
the tympanum is spanned by three elements, the stapes, the pseudo- 
perculum, and the extracolumella, which collectively constitute the 
so-called columella of this group. It is important to observe that 
the intermediate piece, the pseudoperculum, is developed from the 
posterior wall of the tympanic cavity. In lizards the chain of ear 
bones consists of only two, the stapes and the extracolumella. Of 
the three ear bones in the pig the malleus is composed of three 
parts, a manubrium corresponding to the extracolumella of lower 
forms, a body representing the articulare, and a membrane bone 
forming at least a part of the processus gracilis. The stapes of the 
pig is homologous with the stapes of lower vertebrates. The incus 
which unites malleus and stapes cannot correspond to the pseu- 
doperculum of lower vertebrates, because it develops from the ante- 
rior instead of the posterior wall of the tympanic cavity. As this 
position is that occupied by the quadrate, the incus is believed to be 
homologous with this bone. It will thus be seen that while the dis- 
tal and proximal ends of the chain of ear bones in mammals and in 
lower vertebrates are homologous, the intermediate members are not, 
being the posterior pseudoperculum in amphibia and the anterior 
incus (quadrate) in mammals. P. 
Otocysts of the Heteropods. — Ilyin? has experimented upon 
Carinaria and Pterotrachea with the view of determining the physio- 
logical value of the otocysts in these mollusks. The otocysts are 
apparently stimulated not as auditory organs but as tactile organs. 
When both organs are removed, the animal is unable to keep itself 
correctly oriented and swims in circles. The presence or absence 
1 Kingsley, J. S. The Ossicula Auditus, Tufts College Studies, No. 6. (Scien- 
tific Series.) pe 
2 Ilyin, P. Das Gehörbläschen als 29 Ago pao bei den Pterotra- 
cheidz, sro r Phys., Bd. xiii, pp. 691-694, 1900. 
