RELATIONSHIP OF AMMOCCETES TO OSTRACODERMS 337 
median depression between the two median dorsal somatic muscles. 
Also, two lateral cornua pass caudalwards from the main frontal mass 
of muco-cartilage over the lateral eyes, forming the well-known wedge 
which separates the dorsal and lateral portions of the dorso-lateral 
somatic muscle. In fact, similarly to what we find in the branchial 
region, the muco-cartilaginous covering can be traced with greater 
or less completeness only in those parts which are not covered by 
somatic muscles. 
In Fig. 184, A, B, C, this striking muco-cartilaginous head- 
shield, both dorsal and ventral, is shown. Seeing that the upper lip 
wraps round the lower one on each side, and that this most ventral 
edge of the upper lip contains muco-cartilage, as is seen in Fig. 117, 
the dorsal head-shield of muco-cartilage ought, strictly speaking, to 
extend more ventrally in the drawings. I have curtailed it in order 
not to interfere with the representation of the lower lip and tentacu- 
lar muco-cartilages. 
From what has been said, it follows that the past history of the 
skeletal covering of the whole head-region of Ammoccetes, both 
frontal and occipital, can be conjectured by means of the ontogenetic 
history of the foremost myomeres. 
Dohrn and all other observers are agreed that during the develop- 
ment of this animal a striking forward growth of the foremost somatic 
myomeres takes place, so that, as Dohrn puts it, the body-muscula- 
ture has extended forwards over the gill-region, and at the same 
time the gill-region has extended backwards. It is therefore prob- 
able that in the ancestral form the myotomes, innervated by the first 
spinal nerves, immediately succeeded the branchial region. Judging 
from Ammoccetes, the forward growth was at first confined to the 
dorsal region, and therefore invaded the dorsal head-plate, the ventral 
musculature being distinctly a later growth. With respect to this 
dorsal part of the myotomes, the first myotome is originally situated 
some distance behind the auditory capsule, and then grows forward 
towards the nasal opening; the lateral part, according to Hatschek, 
grows forward more quickly than the dorsal part, and splits itself 
above and below the eye into a dorso-lateral part, which extends up 
to the olfactory capsule, and a ventro-lateral part (m. lateralis capitis 
anterior, superior, and inferior), thus giving rise to the characteristic 
appearance of the muco-cartilaginous head-shield of Ammoccetes. 
According, then, to the extent of the growth of these somatic 
: Z 
