72 
no doubt on its surface, and the olfactory pits, have been 
lost, together with the cerebral ganglia. Directly under the 
neuropore lies the anterior end of the notochord and at 
the left and the right of the latter the first pair of somites 
(the “mandibular’” somites of VAN WYHE, the “collar- 
cavities’ of MACBRIDE), corresponding to the coelomeso- 
blast of the first segment, or peristomium, of Annelids. 
All the somites develop regular and permanent myotomes, 
from the first onwards. The first pair sends out a prolongation 
into the prostomium which itself has no primordial meso- 
derm, while at the same time the fore-end of the notochord 
secondarily grows out into the snout. These “head-or 
rostral prolongations” of the first pair of somites correspond 
to the praemandibular cavities of Craniates. Paired gill- 
pouches are formed each between two somites. The first 
pair appears in front of the first pair of somites, under 
the neuropore. The left of them acquires an opening, known 
as HATSCHEK’s pit, to the exterior, the right one gives rise 
to the praeoral head cavity. Of the second pair, between 
the first and the second pair of somites, the left one becomes 
the larval mouth (afterwards the velar opening), the right 
one the so-called club-shaped gland which also opens to 
the exterior but afterwards disappeais. Only secondarily, 
after the “critical stage”, does the eumetamerism of gill-slits 
and somites get lost. The endostyle originates under the 
mandibular segment, in front of the mouth. 
Spinal gang ia and also head ganglia are not clearly recog- 
nizable, auditory vesicles are absent. Dorsal and ventral spinal 
nerves do not unite. There is no distinct difference between 
the nerves of the anterior segments (corresponding to the head 
segments of Craniates) and those of the trunk, both being of 
mixed character. In the region of the former the dorsal roots 
show a strong plexus-formation and a longitudinal plexus also 
supplies the gills. It is especially by the strong fourth nerve, 
evidently comparable to the vagus in Craniates, that thís 
branchial plexus communicates with the medulla and 
the insignificant brain-vesicle which corresponds to the 
epichordal-deuterencephalon of Craniates. 
he asymmetrical origin cf the mouth no doubt gives us the 
key to the interpretation of the larval asymmetry, although 
we shall not indulge now in speculations in this direction. 
Head of Petromyzon. — In Petromyzon part of the surface 
of the prostomium has been annexed by the medullary tube, 
