GLOSSARY. 



tive body-cavity of embryo animals ; believed to represent the 

 fact that their ancestors belonged to that low stage of animal 

 structure found in the Caelenterata, where the body-cavity and 

 stomach are identical. 



Azygous (Gk. d, without, and ^vyiv, yoke, cross-bar joining two 

 sides). — Unpaired; a term applied to structures in the middle 

 of the body. ■ Keally they are composed of two halves, so it is 

 much better to refer to them as median structures. 



Biology (Gk. ;8£os, life, and \6yoi). — The study of living bodies, in- 

 cluding both- plants and animals. 



Blastoderm (Gk. (SX^cttos, germ, and Sipiia, skin). — The developing 

 germ or embryo of animals that have eggs containing yolk ; so 

 called because it lies flat on the top of the yolk, like a thin skin. 



Budding.— The asexual formation of a new animal by a simple pro- 

 cess of growth from the body of the old one. The outgrowth, 

 when it has developed the various parts of a new animal, breaks 

 ofi from the other. Colonial animals are formed by repetitions 

 of this process. 



Caecum, pi. Caeca (Lat. ciecms, blind). — A bag-like branch of any 

 cavity ; i.e. a sac with what we call metaphorically a blind end, 

 just as we speak of a hlind street, meaning one with no outlet. 

 Cffica are attached to the alimentary canal in many animals ; 

 when they occur in invertebrates there are usually several of 

 them ; but in the higher vertebrates there is only one. . 



Cell. — See Plastid. 



Cloaca (Lat. a sewer). — This name is applied to the common passage 

 which serves for the ejection of both excretory and generative 

 products in some animals, e.g. birds and reptiles. 



Coelom or Body-Cavity (Gk. koiXos. hollow).— Iu nearly all animals 

 the internal organs are contained in a common cavity : this is 

 called the coslom or body-cavity. See p. 40. 



Ccenosarc (Gk. Koivi^, shared in common, and aap^, flesh). — The 

 tissue which unites the unit organisms of a colonial animal. In 

 'many kinds it becomes hard, and forms a sort of stalk on which 

 they seem to be borne, e.g. corals. 



Condyle (Gk. k4j/5iiXos, knuckle). — The Jomt (occipital condyle, from 

 Lat. occiput, the head) by which the skull is set on to the spinal 

 column ; double in amphibia and mammalia, single in reptiles 

 and birds (Sauropsida). 



Differentiation (Lat.d/Jcrmtia, difference).— The gradual establish- 

 ment of different parts in tbe course of growth. 



D istal (Lat., from dislo, to be distant); At the far end : thus the hand, 

 is at the distal end of the arm. i 



Diverticulum (Lat., a way turning oH in another direction). A 

 passage or chamber branching off from tbe main portion of any 

 cavity. . 



Ectoderm (Gk. e/cT6s, outside, and S^p/ia, skin). — The name given, 

 to the outside layer of the body in those comparatively simple 



