RELATIONSHIP OF AMMOCCETES TO OSTRACODERMS 337 



median depression between the two median dorsal somatic muscles. 

 Also, two lateral cornua pass caudal wards 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. 134, 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 Aminoccetes, 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 Animoccetes, 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 Amniocuetes. 



According, then, to the extent of the growth of these somatic 



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