A well-marked trochosphere is formed by the development of an equatorial ciliated band; and subsequently, by the disproportionate growth of the lower hemisphere, the trochosphere becomes a veliger.
B, The diblastula has become a trochosphere by the development of the ciliated ring y r (optical section).
C, Side view of the trochosphere with commencing formation of the foot.
D, Further advanced trochosphere (optical section).
E, The trochosphere passing to the veliger stage, dorsal view showing the formation of the primitive shell-sac.
In development they pass through the typical trochosphere and veliger stages provided with boat-like shell.
- Embryo of Limnaeus stagnalis, at a stage when the Trochosphere is developing foot and shell-gland and becoming a Veliger, seen as a transparent object under slight pressure.
The foot now protrudes below the mouth, and the post-oral hemisphere of the trochosphere grows more rapidly then the anterior or velar area.
We may now revert briefly to the internal organization at a period when the trochosphere is beginning to show a prominent foot growing out from the area where the mid-region of the elongated blastopore was situated, and having therefore at one end of it the mouth and at the other the anus.
In development some Lamellibranchia pass through a free-swimming trochosphere stage with preoral ciliated band; other fresh (I) ' Av B m 10 ad 3r ?/ a.: a il J FIG.
The early larva of Anodonta is not unlike the trochosphere of other Lamellibranchs, but the mouth is wanting.
Other Lamellibranchs exhibit either a trochosphere larva which becomes a veliger differing only from the Gastropod's and Pteropod's veliger in having bilateral shell-calcifications instead of a single central one; or, like Anodonta, they may develop within the gill-plates of the mother, though without presenting such a specialized 210 1P -' 1 °* larva as the glochidium.
D, E, F, Trochosphere stage, D fp, Pore in the foot (belonging mf, The mantle-flap or limbus to the pedal gland?).
B, The Gastrula has become a Trochosphere by the development of the ciliated ring vr (optical section).
C, Side view of the Trochosphere with commencing formation of the foot.
D, Further advanced Trochosphere (optical section).
E, The Trochosphere passing to the Veliger stage, dorsal view showing the formation of the primitive shell-sac.
- Free-swimming Larva (Mailer's Larva) of a Polyclad Planarian to illustrate the trochosphere-hypothesis of the origin of Platyelmia.
The former hypothesis with its variants may be called the Trochosphere-hypothesis, the latter the Gastraea-hypothesis.
The Trochosphere-hypothesis (2), (3) is based chiefly on the occurrence in certain Polyclad Turbellaria, of a larval form (Miiller's larva) which is comparable to a certain stage (pro-trochula) in the development of the Trochosphere-larva.
This Trochosphere is the characteristic larva of Mollusca, Annelida T OT (After Abbott, T5ky5 Zool.
The importance of this resemblance between the Polyclad larva and the Trochosphere-larva of higher invertebrates is increased if the widely adopted V Ep (After F.
The external surface of the trochosphere is formed of a number of ciliated test-cells.
This little creature, which has many of the features of a Trochosphere larva, swims about at the surface of the sea for about a month and grows rapidly.
From the trochosphere stage the free larvae pass into that of "veligers."
Most observers consider that Actinotrocha is a highly modified Trochosphere, and this would give it some claim to be regarded as distantly related to the Entoproct Polyzoa and to other groups which have a Trochosphere larva.