THE EVIDENCE OF THE RESPIRATORY APPARATUS 173 
body-muscles, which extend dorsally as far forward as the nose and 
are developed embryologically quite distinctly from the others, being 
formed as muscle-plates (Kastchen). On the other hand, the cranial 
somatic muscles are the eye-muscles, the formation of which resembles 
that of the visceral muscles, and not of the spinal somatic. Their 
direction is not longitudinal, but dorso-ventral; they cannot, in my 
opinion, be referred to the somatic trunk-muscles, and must, therefore, 
form a separate group to themselves. Thus the striated musculature 
of the Ammoccetes must be divided into (1) the visceral muscles ; 
(2) the longitudinal somatic muscles ; and (3) the dorso-ventral somatic 
muscles. Of these the Ist, on the view just stated, represent the 
original appendage-muscles ; the 2nd belong to the spinal region, and 
will be considered with that region; the 3rd represent the original 
segmental dorso-ventral somatic muscles, which are so conspicuous 
in the musculature of the Limulus and the scorpion group. 
The discussion of this last statement will be given when I come 
to deal with the prosomatic segments of Ammoccetes. I wish, here, 
simply to point out that van Wijhe has shown that the eye-muscles 
develop from his 1st, 2nd, and 3rd dorsal mesoblastic segments, and 
therefore represent the somatic muscles belonging to those segments, 
while no development of any corresponding muscles takes place in 
the 4th, 5th, and 6th segments; so that if the eye-muscles represent 
a group of dorso-ventral somatic muscles, such muscles have been 
lost in the 4th, 5th, and 6th segments. The latter segments are, 
however, the glossopharyngeal and vagus segments, the branchial 
musculature of which is derived from the ventral segments of the 
mesoderm. In other words, van Wijhe’s observations mean that the 
dorso-ventral somatic musculature has been lost in the branchial or 
mesosomatic region, while the dorso-ventral appendage musculature 
has been retained, and that, therefore, the mode of respiration in 
Ammoccetes more closely resembles that of Limulus than of Scorpio. 
In addition to these branchial muscles, another and very striking 
set of muscles is found in the respiratory region of Ammoccetes—the 
so-called tubular muscles. These muscles are of great interest, but 
as they are especially connected with the VIIth nerve, their con- 
sideration is best postponed to the chapter dealing with that nerve. 
Also, in connection with the vagus group of nerves, special sense- 
organs are found in the skin covering this mesosomatic region, the 
so-called epithelial pit-organs (Zp. pit., Fig. 71). They, too, are of 
