THE PROSOMATIC SEGMENTS OF AMMDCCETES 309 



The Tubular Muscles. 



The only musculature innervated by the trigeminal nerve which 

 remains for further discussion, consists of those peculiar muscles found 

 in the velum, known by the name of striated tubular muscles. This 

 group of muscles has already been referred to in Chapter IV., dealing 

 with respiration and the origin of the heart. 



It is a muscular group of extraordinary interest in seeking an 

 answer to the question of vertebrate ancestry, for, like the thyroid 

 gland, it bears all the characteristics of a 

 survival from a prevertebrate form, which 

 is especially well marked in Ammoccetes. 

 I have already suggested in this chapter 

 that the homologues of these muscles are 

 represented in Limulus by the veno-peri- 

 cardial group of muscles. I will now 

 proceed to deal with the evidence for this 

 suggestion. H^^S B 



The structure of the muscle-fibres is 

 peculiar and very characteristic, so that 

 wherever they occur they are easily recog- 

 nized. Each fibre consists of a core of 

 granular protoplasm, in the centre of which 

 the nuclei are arranged in a single row. 

 This core is surrounded by a margin of 

 striated fibrillse, as is seen in Fig. 122. 

 Such a structure is characteristic of various 

 forms of striated muscle found in various 



invertebrates, such as the muscle-fibre of mollusca. It is, as far as 

 I know, found nowhere in the vertebrate kingdom, except in Amrao- 

 ccetes. At transformation these muscles entirely disappear, becoming 

 fattily degenerated and then absorbed. 



For all these reasons they bear the stamp of a survival from a 

 prevertebrate form. This alone would not make this tissue of any 

 great importance, but when in addition these muscles are found to be 

 arranged absolutely segmentally throughout the whole of the branchial 

 region, then this tissue becomes a clue of the highest importance. 



As mentioned in Chapter IV., the segmental muscles of respira- 

 tion consist of the adductor muscle and the two constrictor muscles 



Fig. 122. — A 



Muscle-fibre 

 ccetes. 



Tubular 

 of Ammo- 



A, portion of fibre seen longi- 

 tudinally ; B, transverse 

 section of fibre (osmic pre- 

 paration) ; tbe black dots 



are fat-globules. 



