THE EVIDENCE OF THE RESPIRATORY APPARATUS I 73 



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 1st, 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 Vllth 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 ~(Ep. pit, Fig. 71). They, too, are of 



