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SIDNEY ¥. BARHER. 
tions of the nervous system is secondarily established. The 
existence of two cephalic invaginations in Dentalium in no 
way impairs the comparison which has just been instituted 
between it and Loxosoma, where a single invagination 
occurs. Nothing could show this more clearly than the 
manner in which the brain develops in some Pteropods 
(Hyaleacea), described by Fol; this may be explained by the 
quotation of a few lines from Balfour ( f Comp. Emb.,’ i, 
p. 227) : “ A disc-like area appears in the centre of the velum, 
which soon becomes nearly divided into two halves. From 
each of these there is formed by invagination a small sack. 
The axes of invagination of the two sacks meet at an angle on 
the surface. The cavities of the sacks become obliterated ; 
the sacks themselves become detached from the surface, fuse 
in the middle line, and come to lie astride of the oesophagus.” 
This mode of the development of the brain forms a most 
instructive transition from Dentalium to Loxosoma. In 
Dentalium the two halves of the brain are separate in- 
vaginations, extending inwards for a considerable distance 
parallel to the long axis of the embryo. In the Pteropods 
just described, the axes of invagination are inclined to one 
another on the surface, their apertures being situated close 
together ; and by supposing the invagination in the latter case 
to encroach on the middle line, the two apertures would fuse, 
and we should have precisely the same arrangement as that of 
Loxosoma represented in fig. 48. 
It seems to me that if we once admit the nervous nature of 
the parts which I have described as such in Loxosoma, the 
identification of the surfaces in the Entoprocta is no longer 
a matter of doubt. The dorsal organ, if a ganglion at all, 
must be supra-oesophageal, from its position, from the mode of 
its development, and from the fact that it is provided with 
paired eyes (doubtless true cephalic eyes inherited from an 
ancestor common to the Trochosphere group, and not 
accessory eyes like those developed on other parts of the body 
in many Ectoproctan larvae). 
Caldwell (34) has recently suggested a view of the body plan 
