332 THE ORIGIN OF VERTEBRATES 
vary very much in shape, as is seen by the comparison of Tre- 
mataspis and Auchenaspis with Cephalaspis and Eukeraspis, and 
yet, undoubtedly, all these forms belong to a single group, the 
Osteostraci. 
The conception that Ammoceetes is the solitary living form allied 
to this group affords a clue to the meaning of this variation of 
shape, which appears to me to be possible, if not indeed probable. 
There is a certain amount of evidence given in the development 
of Ammoccetes which indicates that the branchial region of its 
ancestors was covered with plates of muco-cartilage as well as the 
prosomatic region. 
The evidence is as follows :— 
The somatic muscles of Ammoccetes form a continuous longi- 
tudinal sheet of muscles along the length of the body, which are 
divided up by connective tissue bands into a series of imperfect 
segments or myotomes. This simple muscular sheet can be dissected 
off along the whole of the head-region of the animal, with the 
exception of the most anterior part, without interfering with the 
attachments or arrangements of the splanchnic muscular system in 
the least. The reason why this separation can be so easily effected 
is to be found in the fact that the two sets of muscles are not 
attached to the same fascia. The sheet of fascia to which the 
somatic muscles are attached is separated from the fascia which 
encloses the branchial cavity by a space (cf. Figs. 63 and 64) filled 
with blood-spaces and cells containing fat, in which space is also 
situated the cartilaginous branchial basket-work. These branchial 
bars are closely connected with the branchial sheet of fascia, and 
have no connection with the somatic fascia, their perichondrium 
forming part of the former sheet. Upon examination, this space 
is seen to be mainly vascular, the blood-spaces being large and 
frequently marked with pigment ; but it also possesses a tissue of its 
own, recognized as fat-tissue by all observers. The peculiarity of 
the cells of this tissue is their arrangement ; they are elongated cells 
arranged at right angles to the plates of fascia, just as the fibres of 
the muco-cartilage are largely arranged at right angles to their 
limiting plates of perichondrium. These cells do not necessarily 
contain fat; and when they do, the fat is found in the centre of each 
cell, and does not push the protoplasm of the cell to the periphery, 
as in ordinary fat cells. 
