132 THE VOYAGE OF H.M.S. CHALLENGER. 



Adaptation of the intrinsic muscle-apparatus to the requirements of the pes. — It is a 

 question about which there cau be no dispute that the number and arrangement of the 

 intrinsic pedal muscles are in a great measure dependent upon the special functions 

 required of the foot. It is disappointing, however, to find so little clear proof of this in 

 the mammalian pes. 



Let us take three well-marked examples of burrowing animals from different orders, 

 viz., the Echidna, the Bathyergus, and the Badger, and compare the disposition of the 

 intrinsic muscles of their feet. The result is found to be highly unsatisfactory. In the 

 Echidna the only layer well represented is the plantar layer. Each digit is supplied with 

 an adductor. Flexores breves on the other hand are only found in connection with the 

 small hallux, the index and the minimus and abductores in connection with the hallux 

 and index. In the Bathyergus the flexores breves are alone developed — one being 

 furnished to each toe. Not a trace of adductores or of abductores exists. In the 

 Badger all three layers are well represented ; thus there are three adductors, five flexores 

 breves, and five abductors. 



If we next contrast the feet of four aquatic animals, viz., the Otter, the Beaver, the 

 Ornithorhynchus, and the Walrus we obtain equally unsatisfactory results. In the Otter 

 the intrinsic pedal muscles meet almost all the requirements of the typical trilaminar 

 arrangement. Each digit is supplied with three muscles, with the exception of the annularis 

 which wants an adductor. In the Ornithorhynchus the digits are also well furnished with 

 intrinsic muscles, but these are arranged upon a different plan, and there is great difficulty 

 experienced in separating the flexores breves from the dorsal interossei. The abductor 

 hallucis, and the flexor brevis minimi digiti are absent ; and further, the second and fourth 

 dorsal interossei are developed into approximators. In the Beaver and Walrus, on the 

 other hand, the plantar layer is only represented by a feeble adductor hallucis. With 

 regard to the intermediate layer, the Walrus possesses a distinct flexor brevis for each toe, 

 whereas in the Beaver the flexores breves are restricted to the index, medius, and annularis. 

 The disparity in the dorsal layer is even more marked. In the Walrus there is a powerful 

 abducting apparatus connected with the minimus, a very rudimentary abductor hallucis, 

 and one questionable dorsal interosseus present, viz., the third. In the Beaver, the 

 abducting apparatus of the minimus is feeble in the extreme ; the abductor hallucis is well 

 developed, and there are two well-marked dorsal interossei, viz., the first and the second. 



In the dissection of these aquatic feet I was struck with the uniform feeble develop- 

 ment of the abducting apparatus of the marginal digits. I fully expected that these 

 muscles would be largely developed for the purpose of expanding the web. In the 

 Walrus alone is the abductor minimi digiti well marked. In the Beaver it is very feeble, 

 and in the Ornithorhynchus and Otter it is only represented by the metatarsal portion, viz., 

 the abductor ossis metatarsi minimi digiti. Then with regard to the abductor hallucis : 

 in the Ornithorhynchus it is absent, in the Walrus and Otter it is very feeble, and in the 



