THE PROSOMATIC SEGMENTS OF AMMOCGTES 309 
Tue 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. 
The structure of the muscle-fibres is A 
peculiar and very characteristic, so that 
wherever they occur they are easily recog- Pe: aod) eee 
nized. Each fibre consists of a core of  jusctm-rrere or AmMo- 
granular protoplasm,in the centre of which — ca@7ss. 
the nuclei are arranged in a single row. A, portion of fibre seen longi- 
This core is surrounded by a margin of  ‘4dinally; B, transverse 
; F a . ; section of fibre (osmic pre- 
striated fibrille, as is seen in Fig. 122.  pavation); the black dots 
Such a structure is characteristic of various are fat-globules. 
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 Ammo- 
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 
