STRUCTURE AND DEVELOPMENT OP LOXOSOMA. 
305 
each side of the oesophagus. Even at a late stage, each pos- 
sesses its own opening to the exterior, although early in their 
development the two tubes become connected by a median 
bridge of subepithelial epiblast cells. Eventually their lumina 
disappear, and they are either partially or completely converted 
into the nervous tissue of the brain, Kowalevsky inclining to 
the latter hypothesis. The manner in which the median com- 
missure connecting the two halves of the ganglion is developed 
has not been satisfactorily followed. In the young Den ta- 
li um the brain is a transversely elongated structure in front 
of the oesophagus, consisting of a central fibrous core sur- 
rounded by ganglion-cells. The similarity between this 
ganglion and the brain of the larva of L. Leptoclini or the 
ganglion of the adult Loxosoma, is so striking that I have 
reproduced Kowalevsky's fig. 92 in my own fig. 23 for com- 
parison with figs. 50 and 51 of the brain of the larva of 
L. Leptoclini, and with fig. 16 of the ganglion (sub- 
oesophageal) of the adult L. Tethyse. When it is remem- 
bered that neither of these organs in Loxosoma has hitherto 
received the interpretation suggested in this paper, it will 
probably be admitted that the histological characters of the 
structure which is undoubtedly the brain of Dentalium, 
confirm very strongly the view that the above-mentioned 
organs of Loxosoma are similarly nervous in character. 
The pedal ganglia of Dentalium develop as paired thicken- 
ings of the epiblast at the sides of the foot, not far behind the 
oesophagus. They originate quite independently of the cephalic 
ganglion, with which a secondary connection is established ; 
and this, as Kowalevsky points out, is in all probability cha- 
racteristic of the development of the Mollusca in general. 1 
In both Loxosoma and Dentalium the brain originates 
by a process of invagination, the suboesophageal ganglia (pedal) 
as paired thickenings of the epiblast, and not, like the brain, 
as invaginations, whilst the connection between the two por- 
1 In the same way, in Lumbricus trapezoides (Kleinenberg), the 
brain and ventral cord originate independently and become secondarily con- 
nected (Balfour, * Comp. Emb.,’ vol. ii, p. 336). 
