THE FOOT. 



The morphological value of t he arm s of the Cephalopoda has been the 

 subject of an extended controversy among morphologists. Some have maintained 

 that the siphon represents the entire foot and that the arms are appendages of 

 the head: others have held that the siphon is only the posterior part of the 

 foot and that the arms represent its anterior portion. The fact that the arms 

 appear along the ventral portion of the sides of the embryo and later move 

 forv^ard to their final position around the "mouth and that the structure of the 

 arms indicates that such a migration has occurred led finally to the general 

 acceptance of the latter view. 



The Arms. In the adult squid, five pairs of arms are borne by a circular 

 mass of muscle that encircles the mouth , encloses the pharynx , and is attached 

 to the ventral surface of the skull. The tentacle, the fourth arm on each side 

 (numbering from above downward) , is longer than the others and has an 

 expanded extremity, the "club", which bears four rows of suckers and which 

 is supported by a long retractile stalk , the peduncle. The four remaining pairs 

 of arms, the sessile arms, taper regularly from the base to the apex and bear 

 two rows of suckers along their entire oral surfaces. In section, the first, 

 second , and fifth arms are trapezoidal ; the third is oval. The oral surface of 

 each sessile arm is bordered by narrow engrailed membranes whose points are 

 formed by the tips of small muscular buttresses which alternate with the suckers 

 and support the membranes. When the muscles of the buttresses are relaxed 

 the fold of skin which forms the membrane is drawn down to the surface of 

 the arm so that there remains, as the only trace of the membrane, a line of 

 fleshy protuberances, the relaxed buttresses. More substantial membranes border 

 the outer mesial angle of the first arm , the lateral outer angle of the second , 

 the single outer angle of the third , and both outer angles of the fifth arm. 

 The outer lateral membrane of the fifth aim is very broad at its base where it 

 stretches across outside the fourth aim and unites with the membrane of the 

 third arm. The sheet thus formed by the proximal part of the membranes of 

 the third and fifth arms is attached dorsally to the ventral edge of the capsule 

 of the eye and forms part of the outer wall of a large sac which lies between 

 this sheet and the inner side of the eye on one side , and the bases of the third 



