DEVELOPMENTAL HISTOEY OF THE MOLLTJSCA. 
29 
of a very peculiar boat-like form, and the velum is placed near the middle of it like a 
pair of paddle-wheels. In fig. 11 such an embryo is seen from above. In fig. 12 the 
outline of the same is drawn, in order to show the two muscular bands ( m ) which come 
into view with a deeper focus. One of these is seen at fig. 12, a as displayed by a 
Hartnack’s 10 a immersion. 
Polycera and Tethys. — In Plate 10 figures are given of the embryos of Polycera qua- 
drilineata and of Tethys at a time when they give evidence of a primitive invagination. 
In Plate 10. figs. 1, 2, 3, three views are given of the embryo of Polycera , showing 
the long groove of invagination, similar to the condition of Plate 9. fig. 5. The outer 
cells are more transparent than the inner. 
In Plate 10. figs. 10, 11, an early condition of the yelk-division of Tethys is pre- 
sented. I am not able to figure the steps of invagination in this molluscan embryo ; 
but the stage illustrated in figures 12 to 16 gives some evidence of the remains of an 
aperture of invagination (i). 
Plate 10. figs. 12 & 16 are left lateral views ; fig. 13 is a right lateral view; fig. 14 
an aboral view ; fig. 15 an oral view of the same embryo. 
These embryos are interesting to compare with the early stages of Aplysia described 
in the preceding section of this communication. Especially the two mantle-rudiments 
( mn ) are to be noted as making here an early appearance as in Aplysia *. 
The remaining figures of Polycera embryos, viz. Plate 10. figs. 4-9, are chiefly of 
interest for the sake of comparison with the corresponding “ veliger ” of Aplysia. 
Plate 10. figs. 4 & 5 represent two views of a young stage in which the shell is just 
beginning to appear. The curiously dark-coloured mass (q) I am not able to explain. 
In Plate 10. fig. 5 it is seen that one otolith has formed before the other. 
Plate 10. figs. 6-9 represent fully formed embryos nearly ready to escape from their 
capsules. Fig. 6 is a right lateral view ; fig. 7 a back view ; fig. 8 a front view ; 
fig. 9 a three-quarters profile view. A comparison of these figures with those of the 
veliger of Aplysia minor will show the close correspondence even to the fusiform 
muscle-cells which pass from the perianal mass (dry) to the body-wall. 
Neritina Jluviatilis. — Plate 9. figs. 1-8 represent early stages in the development of 
this mollusk. It is abundant in the river Thames at Godstow, near Oxford. After 
searching in various spots I at last succeeded in obtaining the egg-capsules in quantity 
from stones at the bottom of the river in front of the little inn near Godstow Priory. 
The stones in this part of the river are covered with the broken remains of the capsules 
deposited and hatched-out in former years. The fresh ones in the month of May stud 
these stones in great numbers, each capsule being about the size of a large pin’s head. 
The specimens obtained thence in 1873 were transferred to the histological laboratory of 
* March 7th, 1875. — From observations made in December 1874 and communicated to me by my friend 
Mr. F. M. Balfour, of Trinity College, Cambridge, it seems that the cells mn have not, as I supposed, the 
same significance as in Aplysia, and that the part marked f in Plate 10. figs. 13 & 16 is not the foot but the 
velum. 
