GLOSSARY 485 



Decapoda (Gr. deka, ten ; podes, feet) : the division of Crustacea, which have 

 ten walking feet ; and also the family of cuttle-fishes which possess ten 

 " arms " or head-processes arising from the " foot " and surrounding 

 the mouth. 



Deric (Gr. deros, skin) : a term used to denote a layer derived from the derma 

 or corium, i.e. mesodermal in origin, as contrasted with ectodermal. 



Derma (Gr. derma, skin) : a term usually applied to denote the deeper layer 

 of the skin, or corium. 



Dibranchiata (Gr. dis, twice ; branchia, gill) : a subclass of the Cephalopoda 

 characterized by the presence of two gills or ctenidia. The foot is sub- 

 divided into eight or ten arms, which surround the mouth. The subclass 

 includes the cuttle-fishes, squids, spirula, and the extinct belemnites. 



Dicynodontia (Gr. dis, twice ; kuon, dog ; odous, tooth) : an extinct order 

 of reptiles having two large teeth in the upper jaw, which resemble the 

 " canine " teeth of a dog. 



Dinichthys (Gr. deinos, strange, terrible ; ichthus, fish) : an extinct genus of 

 fishes belonging to the order Arthrodira. 



Dinophilus T^niatus (Gr. dinos, whirling movement or eddy ; philos, fond ; 

 tainia, band) : a cylindrical worm-like animal characterized by a series 

 of circular ciliated bands, an apical plate, and paired ocelli, belonging 

 to the phylum Trochelminthes. 



Diplostichous (Gr. diploos, double ; stichos, layer) : a term applied to an eye 

 consisting of two layers of cells, e.g. the central eyes of Limulus, in 

 which a single layer of hypoderm or vitreous cells lies in front of the 

 retina. 



Dipnoi (Gr. dis, twice ; pnoe, breath) : the order of fishes comprising Ceratodus 

 Lepidosiren and Protopterus, commonly known as lung-fishes or mud- 

 fishes. Their respiration is aquatic, by means of gills, and aerial by a 

 single sacculated lung, corresponding to the air-bladder of Ganoids 

 and Teleosts. 



Diprosopus (Gr. diprosopos, two-faced) : a double-faced monster, as in 

 Duplicitas anterior. 



Diptera (Gr. dis, twice ; pteron, wing) : an order of the Insecta having a single 

 pair of transparent membranous wings, e.g. bugs, plant lice. 



Dorsal sac : the recess which lies between the velum transversum in front 

 and the superior or habenular commissure behind. Its roof forms the 

 post- velar arch. 



Dytiscus marginalis (Gr. dutes, diver ; dutikos, fond of diving) : a carnivorous 

 water beetle belonging to the group Hydradephaga. 



Echidna (Gr. echidna, adder ; monster) : the spiny ant-eater of Australia, 

 belonging to the order Monotremata, comprising the egg-laying 

 mammals. 



Ectoderm (Gr. ektos, outside ; derma, skin) : the outer layer of cells in a multi- 

 cellular animal. It corresponds to the term " epiblast " of the growing 

 embryo, and gives rise to the epidermis of the adult animal. 



Ectoparietal-eye (Gr. ektos, outside ; L. paries, wall) : the term applied by 

 Patten to " the outer of the two vesicles produced by constriction of 



