Spengelomenia. 473 
morphosis, but neither these nor the ones described by Pruvor (1892) 
enable one to decide upon the form of body of the ancestral type. 
However it is important to note that in the young of Halomenia 
the anterior pedal gland, in process of formation, arises in the mid 
line behind the mouth as it does in the chiton embryo, and this 
suggests, though it does not prove conclusivly, that formerly a creep- 
ing sole existed. 
There has been considerable discussion regarding the subject 
of a foot in the solenogastres; whether the so-called ventral fold is 
the remnant of an ancient creeping surface or a late formation. 
In my opinion the ventral fold is a foot, and probably much reduced, 
as it has disappeared in the C'haetodermatidae; and that it is a foot is 
strongly supported by the fact that it is provided with two sets of 
elands, termed by Hugrecht (1883) the anterior and posterior pedal, 
that are found in the chiton embryo. As I have shown (1911) the 
anterior pedal (the foot gland of KowAuevskı) disappears in the 
chitons at an early age, but before it vanishes it takes up a posi- 
tion similar to that in the solenogastres. In origin also the anterior 
pedal glands are similar in the chitons and the solenogastres. 
Finally there is a cleft in the heavy muscles of the ventral side in 
the mid line in the Chaetodermatidae, and all of these facts appear 
to me to indicate that the ventral fold is a foot, the remnant of a 
more fully developed one in the ancestral type. 
NIERSTRASZ (1908, p. 250) has quoted me as making the claim 
that the “semicircular groove” is the equivalent of the outlet 
of the anterior pedal gland.. What I have claimed is that in 
Limifossor there is a furrow (halbmondförmige Grube of Wiren) 
similar to the one found in Chaetoderma situated beneath the buccal 
sensory plate. It is lined throughout with the spiculose investment 
of the body, and to the base of it powerful muscles attach, and further- 
more it is not the same as the cleft in the ventral somatic muscu- 
lature. In other words I have not claimed that it is in any way 
related to the ventral groove in the Neomenividae. 
In some species of solenogastres several folds exist in addition 
to the usual median one, and in Pachymenia abyssorum there is a 
relativly broad creeping surface, but whether these are additional 
folds that have been secondarily derived from the outlying hypo- 
dermis, and whether in the second case the creeping surface has 
widened in comparativiy recent times is not certain, but if this is 
