NOTES 477 



must lose as much as it receives ; that is, the excretions, including the 

 solid residuum ejected from the intestinal canal, equal the food and drink. 



125 Other names for derm are, cutis, corium, enderon, and true skin ; 

 and for epidermis, cuticle, ecderon, and scarfskin. The derm is often so 

 intimately blended with the muscles that its existence as a distinct layer is 

 not easily made out. 



126 Papillae are scarcely visible in the skin of reptiles and birds. 



127 The animal basis of this structure is chitin, a peculiar hornlike sub- 

 stance found in the hard parts of all the articulated animals. 



128 The shell is always an epidermal structure, even when apparently 

 internal. The horny "pen" of the squid, the "bone" of the cuttlefish, 

 and the calcareous spot on the back of'the slug are only concealed under 

 a fold of the mantle. So the shell of the common unio, or fresh-water 

 clam, is covered with a brownish or greenish membrane, which is the 

 outer layer of the epidermis. Where the mantle covers the lips of a shell, 

 as in most of the large sea snails, or where its folds cover the whole ex- 

 terior, as in the polished cowry, the epidermis is wanting, or covered up 

 by an additional layer. 



129 The pearls of commerce, found in the mantle of some mollusks, are 

 similar in structure to the shell ; but what is the innermost layer in the 

 shell is placed on the outside in the pearl, and is much finer and more 

 compact. The pearl is formed around some nucleus, as an organic particle, 

 or grain of sand. 



130 \vhen the centrum is concave on both sides, as in fishes, it is said to 

 be amphiccelous ; when concave in front and convex behind, as in croco- 

 diles, it is called proccelous ; when concave behind and convex in front, as 

 in the neck-vertebrae of the ox, it is opisthoccelous. In the last two cases, 

 the vertebras unite by ball-and-socket joints. 



131 Whether the skull represents any definite number of vertebrae was long 

 under discussion. We cannot speak of " cranial vertebrae " in the same 

 sense as " cervical vertebrae." The most that can be said is that in a gen- 

 eral way the skull is homologous to part of the vertebral column. 



132 A few have but one pair, the whale and siren wanting the hind pair ; 

 while some have none at all, as the snakes and lowest vertebrates. In 

 land animals, the posterior limbs are generally most developed ; in aquatic 

 animals, the anterior. Dr. Wyman contends that the limbs are tegumen- 

 tary organs, and attached to the vertebral column in the same sense that 

 the teeth are attached to the jaws. Other theories are that they originate 

 from gill arches (Gegenbaur) or that they are remains of a once continu- 

 ous lateral fin (Thacher). 



133 The first trace of muscular tissue is found' in the stem of vorticella 

 an infusorian. In hydra we find neuro-muscular cells, and the jellyfishes 

 have muscular tissue. 



134 The muscles of some invertebrates, as spiders, are yellow. 



