64 Nils Hj. Odhner. 
eS 
dorsal side, where a protecting coat is acquired, and as the muscular 
layer is thinner than that of the foot, this furrow is best adapted 
to the development of breathing organs. 
Though this process is entirely hypothetical, its probability is 
borne out by the actual organization of Gymnomenia, which at least 
shows that the foot musculature of Chiton, which is homologous 
with the shell muscles of Fissurella and the columellar muscle of 
other Gastropoda, has most probably developed from the septal 
muscles of Solenogastres or allied forms with a similar construction. 
Against this hypothesis it may be argued that the ventral side 
in the Solenogastres, except the furrow, is as strongly cuticularized 
as the remaining surface of the body, whilst in Chiton the sole 
differs from the upper side in being entirely bare, and thus more 
similar to the ventral fold of a Solenogastre. This objection is quite 
justified, as it shows that we must date the origin of the Mollusca 
from forms more ancient than the Solenogastres in the respect that 
no cuticula yet existed on their ventral side; this is an acquisition 
peculiar to the Solenogastres. The ventral furrow has certainly 
arisen as a consequence of this cuticularization and the presence of 
the pedal gland; thus it is absent in forms which lack this (Chaeto- 
derma, Chiton). \ts homologon is probably to be sought in the 
byssus furrow of the Lamellibranchiata. The fold or folds with- 
in the groove are undoubtedly caused by the ducts of glands de- 
bouching into it, just as is the case with the folds of the byssus 
cavity. 
It may be questioned whether the primitive condition is a 
single fold or numerous ones. The last-mentioned alternative seems 
according to THIELE (1902) to be most probable, and Neomenia 
and other genera with many folds are thus most primitive. It seems, 
however, to be more natural to assume that the primitive condition 
is represented by forms such as Nematomenia, where every trace 
of ventral folds is absent. From the latter the fold-bearing Soleno- 
gastres may have developed in different directions, one acquiring 
many, the other a single fold, each group thus having originated 
separately from the other. The presence of the two ventral stripes 
of glands has certainly caused a separation of the ventral ciliated 
epithelium from that of the lateral sides of the body, which has 
been cuticularized, and the number of efferent canals from the pedal 
glands has caused the further differentiation of the median ventral 
furrow into a monoplicate or polyplicate zone. 
