GLOSSARY 489 



Hypostome (Gr. hupo, under ; stoma, mouth) : the upper lip or " labrum" 

 of certain Crustacea, e.g. Trilobita. 



Ichthyopsida (Gr. ichthus, fish ; opsis, appearance) : the division of vertebrata 

 which comprises the fishes and amphibia ; also known as branchiate 

 vertebrates. 



Ichthyopterygia (Gr. ichthus, fish ; pterux, wing or fin) : an extinct order 

 offish-like reptiles, which includes the Ichthyosauri. 



Ichthyosaurus (Gr. ichthus, fish ; saura, lizard) : an extinct order of reptiles 

 belonging to the Mesozoic period. 



Isopoda (Gr. isos, equal ; pous, foot) : an order of the Crustacea in which the 

 feet are approximately equal in size and appearance, e.g. Asellus, 

 Sphceroma. The eyes are compound and usually sessile ; in some 

 examples, however, they consist of a collection of simple eyes. 



Jurassic System or Period : the oolitic and lias deposits found in the Jura 

 Mountains and stretching across England from Yorkshire to Dorset. 

 It belongs to the Mesozoic period and contains remains of marine 

 animals such as the ammonites and large Saurian reptiles. 



Kainozoic (Gr. kainos, recent ; zoe, life) : the tertiary period in geology, in 

 which the remains of animals and plants closely resemble living 

 species. 



Kiaeraspis (Gr. apis, shield) : a primitive Palaeozoic fish belonging to the 

 Order Cephalaspidomorphi of the branch Ostracodermi. 



King-crab (Limulus) : a marine arachnid belonging to the order Xiphosura. 

 It is characterized by a large dorsal-shield and plate-like " book gills." 



Labium (L. lip) : term used to denote the lower lip of articulate animals. See 

 Labrum. 



Labrum (L. lip) : term used to denote the upper lip of articulate animals. 



Labyrinthodontia (Gr. laburinthos, labyrinth ; odous, tooth) : an extinct 

 order of the Amphibia, also named Stegocephala. The term was em- 

 ployed to express the complicated structural appearance of the teeth 

 when seen in transverse section. 



Lamellibranch (L. lamella, plate ; Gr. branchia, gill) : a bivalve mollusc 

 having lamellar gills, e.g. Fresh-water mussel (Anodonta cygnea) : 

 scallop (Pecteri). In some, the larva of which lead a parasitic existence, 

 visual sense-organs are vestigial or absent. In others, e.g. Pecten, highly 

 differentiated eyes are developed in the adult animal round the edge of 

 the mantle. 



Lens (L. lens, lentis, lentil) : the crystalline lens of the eye, so named on account 

 of its being about the size and shape of a lentil seed. There are three 

 principal types of lens, named according to their composition : cuticular, 

 vitreous, cellular. In some instances two or all three of the constituent 

 elements may be combined in the formation of a single lens. 

 Lepas fascicularis, Lepas anatifera (L. lepas, small shell-fish) : cirripede 

 Crustacea, e.g. barnacle, characterized by the fixed condition of the 

 adult animal and by passing through a free-swimming nauplius stage, 

 which is followed by a cypris stage in which the larva is enclosed in a 

 bivalved shell and has both simple (ocellar) and compound eyes. 



