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 Amrnocoetes 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 Amrnocoetes 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 Amrnocoetes 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 lie 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. 



