Evolution of Animals 419 



the apodous and the urodele batrachians, also in the mam- 

 maUan embryo, is wholly or largely, or in reduced and re- 

 stricted degree only, covered with cilia. In the Batrachia the 

 cilia become either partially or wholly shed in later life from 

 formation of an external cuticular covering. Owing doubtless 

 to the relation of the embryo to surrounding food, to a passive 

 viviparous embiyonic stage, and the formation of a cuticular 

 layer as in Batrachia, the mammalian embryo wants cilia. 

 But that it is derived from one that possessed such is evidenced 

 by the different regions or sensitive cavities which show such, 

 and wliich are derived directly or indirectly from the epidermis. 

 The restricted and transitory state seen in the Batrachia has 

 therefore become more circumscribed in Mammalia. 



In the Rotifera, the rhabdocoel Turbellaria, and in most 

 of the Nemertinea the skin is provided with one or more sets 

 of secreting cells, that exude definite surface excretions. In 

 most of the Nemertinea simple subepidermal goblet cells that 

 secrete mucus may be mixed with rod cells {Drepanophorus), 

 or deeply sunk clustered cells may pour out a surface exudate 

 by long ducts (Carinella), or again nematocysts, that remind 

 one of the thread cavities in some Infusoria, may be encoun- 

 tered. Usually therefore an abundant mucous secretion is 

 poured out that may become an enveloping mucous tube in 

 some nemertean land forms. 



In Cyclostomata goblet mucous-secreting cells and nemato- 

 cysts are both developed. The quantity of mucus excreted 

 may be moderate (Petromyzon) or great {Myxine). In the 

 Batrachia apoda deeply sunk mucilage glands occur, along 

 with squirt gland cells that exude an irritant poisonous juice 

 which at least physiologically may be compared with the 

 liquid of the nematocysts above. In the urodele batrachians 

 abundant mucus or slime glands are intermixed with poison 

 glands. As newt-like forms became increasingly terrestrial 

 the slime glands would tend somewhat to dry up unless under 

 special or supraoptimum stimulus. Thus transition would 

 be effected to the mammalian skin where sweat glands, se- 

 baceous follicles, and milk glands are all present, and excrete 

 under appropriate conditions. 



In all of the groups now under consideration the mouth 

 is anterior and mostly subterminal, the simple alimentary 

 canal is continued backward and ends in a subterminal anus 

 in most, though in some rotifers and in turbellarians the canal 

 ends bhndly. The body surface exhibits no definite segmen- 

 tation, though annular constrictions of the muscles and con- 

 nective tissue, as well as the internal disposition of nerves. 



