488 THE PINEAL ORGAN 



Gymnophiona (Gr. gumnos, naked ; ophis, snake) : an order of the Amphibia 



comprising the snake-like Caecilia. 

 Gymnotus (Gr. gumnotes, naked) : the electric eel of South America. 

 Gyrinid^e. (Gr. gureud, run in a circle) : a family of beetles which possess two 



pairs of compound eyes. See Bibionida. 

 Hatteria punctata or Sphenodon, a primitive type of reptile belonging to 



the order Rhynchocephalia, in which the parietal eye is well developed. 

 Helix nemoralis (Gr. helix, twisted or coiled) : land-snail, belonging to the 



order Pulmonata of the Mollusca. The eyes are situated at the apex 



of each posterior or ocular tentacle. 

 Hemiaspida (Gr. hemi, half; ajpis, shield) : an extinct Palaeozoic order related 



to the Xiphosura, and including Bunodes, Hemiaspis, and Belinurus. 

 Hemichorda (Gr. hemi, half ; chorde, string) : a subphylum of the chordata, 



comprising Balanoglossus. An oesophageal diverticulum in the region 



of the neck is regarded as a rudiment of the notochord. The tornaria, 



or larva, has an apical plate bearing two eye-spots. 

 Hemiptera (Gr. hemi, half; pteron, wing) : an order of the Insecta in which 



the anterior wings are membranous at the tip, while the inner part is 



chitinous — e.g. bugs, plant lice, cicadas, lice {Anoplura), wingless 



parasites of mammals. 

 Hexapoda (Gr. hexa, six ; pous, foot) : name applied to the Insecta on account 



of their having six legs. 

 Hirudinea (L. hirudo, leech) : the order of Annelida, which includes the 



leeches. 

 Holocephali (Gr. holos, whole ; kephale, head) : a sub-order of the Elasmo- 



branchs which includes Chimcera and Callorhynchus. 

 Holochroal eyes (Gr. holos, complete ; chroa, surface) : the term used to 



denote a form of compound lateral eye of trilobites, in which the corneal 



surface was continuous ; the ommatidia not being separated as in the 



schizochroal type. 

 Holostomata (Gr. holos, whole ; stoma, mouth) : a division of the gasteropod 



molluscs in which the opening of the spiral shell is entire. 

 Homarus : lobster belonging to the order Decapoda, of the class Crustacea. 

 Hoplocarida (Gr. hoplon, armour ; karis, a small lobster) : a division of the 



Crustacea, comprising the squills, in which the carapace or head-shield 



covers only the anterior thoracic segments. Large stalked eyes are 



borne by the first segment of the anterior region of the head. 

 Hydrodroma (Gr. hudra, water ; dromos, movement, running) : a fresh-water 



mite belonging to the family Hydrachnidae and the order Acarina. It has 



two paired eyes and a central or median eye. 

 Hymenoptera (Gr. humeri, membrane ; pteron, wing) : an order of the Insecta 



characterized by having four membranous wings, e.g. bee, wasp, ant. 

 Hypodermis (Gr. hupo, under ; derma, skin) : the single layer of cells which 



lies beneath the cuticle in invertebrates. It is also known, with reference 



to invertebrate eyes, as the vitreous or corneagen layer. It corresponds 



to the deeper cells of the epidermis of vertebrates and the tall columnar 



cells composing it are spoken of as palisade or epiderm cells. 



