386 NOTES. 



67 Crocodiles are the* only Reptiles whose nostrils open in the throat behind 

 the palate, instead of directly into the mouth-cavity. This enables the Croc- 

 odile to drown its victim without drowning itself; for, by keeping its snout 

 above water, it can breathe while its mouth is wide open. 



68 A rudimentary diaphragm is seen in the Crocodile and Ostrich. 



69 The poison-glands of venomous Serpents and the silk-vessels of Cater- 

 pillars are considered to be modified salivary glands. Birds, Snakes, and 

 Cnrtilaginous Fishes have no urinary bladder. 



70 Since the weight of a full-grown animal remains nearly uniform, it must 

 lose as much as it receives; that is, the excretions, including the solid resid- 

 uum ejected from the intestinal canal, equal the food and drink. 



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

 for epidermis, cuticle, ecderon, and scarf-skin. The derm is often so inti- 

 mately blended with the muscles that its existence as a distinct layer is not 

 easily made out. Even in Infusoria, we find the tunic double, an outside 

 cuticula lined by a soft cortical layer ; and in Jelly-fishes, naturalists distin- 

 guish an ectoderm, endoderm, and mesoderm. 



72 See Fig. 148. Papilla? are scarcely visible in the skin of Reptiles and Birds. 



73 The animal basis of this structure is chitine, a peculiar horn-like substance 

 found in the hard parts of all the articulated animals. 



74 The shell is always an epidermal structure, even when apparently internal. 

 The horny " pen" of the squid, the " bone" of the Cuttle-fish, and the cal- 

 careous 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 epider- 

 mis. Where the mantle covers the lips of a shell, as in most of the large sea- 

 snails, or where its folds cover the whole exterior, as in the polished Cowry, 

 the epidermis is wanting, or covered up by an additional layer. 



75 The pearls of commerce, found in the mantle of some Mollusks, are simi- 

 lar 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. 



76 When 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 Crocodiles, 

 it is called procwlous ; when concave behind and convex in front, as in the 

 neck-vertebra? of the Ox, it is opisthoccelous. In the last two cases, the ver- 

 tebrae unite by ball-and-socket joints. 



77 Whether the skull represents any definite number of vertebra? is still 

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

 sense as "cervical vertebra?." The most that can be said is that in a general 

 way the skull is homologous to part of the vertebral column (B). 



78 A few have but one pair, the Whale and Siren wanting the hind pair ; 

 while some have none at all, as the Snakes arid lowest Fishes. In land ani- 

 mals, the posterior limbs are generally most developed : in aquatic animals, 

 the anterior. Dr. Wyman contends that the limbs are tegumentary 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 continuous lateral fin (Thacher). 



