STRUCTURE AND DEVELOPMENT OF LOXOSOMA. 291 
particular that of the adult, in which can be similarly distin- 
guished oesophagus, stomach (with “liver-cells” ventrally), 
intestine, and rectum, the various parts having exactly the 
same relations to the two limbs of the U-shaped gut in the adult 
and in the embryo alike. Both the mouth and the anus open 
within the circumference of the ciliated ring ( cr .), with respect 
to which the larva is entoproctous, just as is the adult in 
relation to the lophophore. 
Formation of the Sucker, Dorsal Organ, and Vestibule. — At 
the time when the alimentary canal is just completed a series 
of most important changes takes place in the constitution of the 
embryo, involving the appearance of the sucker (the homologue 
of the adult foot-gland), of the “ dorsal organ,” and of the 
vestibule. Fig. 37 is an almost median longitudinal section of 
an embryo at a a period when the sucker and the “ dorsal 
organ” are still in a very early stage of development. The 
oesophagus ( 02 .) is cut medianly, the epiblastic invagination 
from which it has been formed extending as far as the mark 
z. The sucker ( 5 .) is distinctly marked out as a group of 
epiblast cells much higher than their neighbours, and already 
indicated on the exterior by a slight depression of the surface 
of the embryo. The alterations by which the sucker passes 
into its permanent form are very unimportant, the most 
noticeable change consisting in a deepening of the involution, 
and an increase in the number and in the height of the cells 
composing the organ. The sucker, which is retractile, serves 
for the attachment of the embryo to the walls of the brood 
pouch, and this is one of the causes of the fact that it is func- 
tional at so early a stage. The foot-gland is no doubt the homo- 
logue of the larval sucker, with which it is much more similar 
in structure than could be supposed from previous descriptions 
of the adult organ. If my description of the latter is correct 
the main difference between the embryonic sucker and its 
adult representative consists in the elongation of the rounded 
depression, constituting the former, to the open elongated 
groove of the latter; in the adult, however, an additional 
complication has taken place in the separation of the ventral 
