RELATIONSHIP OF AMMOCCETES TO OS TRA CODE RMS **I 



33 



were preserved. Of these, some would indicate the position of blood- 

 vessels, such, for instance, as of the external carotid which traverses 

 this structure ; but the largest and most internal spaces, resembling 

 Eohon's medullary spaces, would represent muscles, being filled up 

 with bundles of the upper lip-muscles. 



The Muco-Caktilaginous Head-Shield of Ammoccetes. 



The resemblance between the structure of the head- shield of 

 Thyestes and the muco- cartilage of Ammoccetes, is most valuable, 

 for muco-cartilage is unique, occurs in no other vertebrate, and every 

 trace of it vanishes at transformation ; it is essentially a character- 

 istic of the larval form, and must, therefore, in accordance with all 

 that has gone before, be the remnant of an ancestral skeletal tissue. 

 The whole story deduced from the study of Ammoccetes would be 

 incomplete without some idea of the meaning of this tissue. So 

 also, as already mentioned, the skeleton of Ammoccetes is incomplete 

 without taking this tissue into account. It is confined entirely to 

 the head-region ; no trace of it exists posteriorly to the branchial 

 basket-work. It consists essentially of dorsal and ventral head- 

 shields, connected together by the tentacular, metastomal, and thyroid 

 bars, as already described. The ventral shield forms the muco-carti- 

 laginous plate of the lower lip and the plate over the thyroid gland, 

 so that the skeleton ventrally is represented by Fig. 118, B, which 

 shows how the cartilaginous bars of the branchial basket-work are 

 separated from each other by this thyroid plate. At transformation, 

 with the disappearance of this muco-cartilaginous plate, the bars 

 come together in the middle line, as in the more posterior portion 

 of the branchial basket-work. 



The dorsal head-shield of muco-cartilage covers over the upper 

 lip, sends a median prolongation over the median pineal eyes and 

 a lateral prolongation on each side as far as the auditory capsules, 

 giving the shape of the head-shield of muco-cartilage, as in Fig. 

 118, C. 



Not only then is the structure of the head-shield of a Cephalaspid 

 remarkably like the muco-cartilage of Ammoccetes, but also its 

 general distribution strangely resembles that of the Ammoccetes 

 muco-cartilage. 



Now, these head-shields in the Cephalaspida 1 and Tremataspidte 



