GLOSSARY. 279 



HydroRcium. A sac attached to the nectocalyx of some of the 



oceanic Hydrozoa. 

 Hydrophyllia. The plates protecting the polypites of the oceanic 



Hydrozoa. 

 Hydrophyton. " The common case of the trophosome by which 



its zooids are connected into a single colony " (Allman). 

 Hydrorhiza, "The proximal end of the hydrophyton, by which 



the colony fixes itself to other bodies" (Allmari). 

 Hydrosoma. The entire colony of the Hydrozoa. 

 Hydrotheca. See Calycle. 

 Hyoid. The bone of the tongue. 

 Hypermetamorphosis. When an insect passes through several 



larval stages. 



Hypoblast. The inner mucous layer of cells of the blastoderm, 

 the eudoderm of the adult ; in the higher animals the latter 



becomes the epithelium. 



Eeum. The small intestine opening into the colon. 

 Ilium. The large pelvic bone on each side of the sacrum. 

 Imaginal disks. Centres of formative tissue in the larva of cer- 

 tain insects, especially Diptera, which give rise to the legs, 



wings, &c. 



Imago. The perfect insect. 

 Incus. One of the bones of the ear. 

 Individual. Physiologically a single form, morphologically an 



entirety of independent beings, the result of the development 



of a single ovum. 

 Individuation (Mivart). The power which differentiates and 



assimilates " all that comes to it (the germ) into a definite and 



predeterminate issue." 

 Infundibulum. A tubular organ found in the Cephalopoda. 



Alan one of the gastric cavities of the Ctenophora. 

 Ingluvies. The crop a dilatation of the oesophagus in birds. 

 Ink-bag. In the Cephalopoda an oval or pyriform glandular 



sac, placed near the liver or within it, and secreting a dark 



fluid. 

 Inquiline or commensal. A tenant, not a parasite ; an animal 



that dwells with, not at the expense of, its host. 



