390 PRINCIPLES OF ANIMAL BIOLOGY 



Myofibril (mi' o ji' hril). One of the contractile threads in a voluntary muscle cell. 

 Myosin {mi' o sin). A common protein in muscle. 



Myotome. One of the segments into which certain muscles are divided. 

 Myriapoda (meer' i ap' o da). A class of Arthropoda having tracheae, one pair of 



antennae, and many unspecialized legs; centipedes and millipedes. 

 Mysis. A genus of Crustacea having all appendages two-branched; also a larval 



stage of other Crustacea in which all appendages are two-branched. 

 Myxedema (miks' e de' ma). A disease whose symptoms are puffy tissues, reduced 



metabolism, and mental depression, caused by deficient thyroid action. 



Nacre. The pearly substance secreted by mollusks upon their shell or other objects. 



Nais. A genus of fresh-water worms, phylum Annelida, subclass Oligochaeta. 



Nasal pit. The ectodermal depression in an embryo which forms much of the nostril. 



Natrix. A genus of snakes. N. rhombifera, N. sipedon, two of the species. 



Natural history. A descriptive account of things in nature, particularly animals and 

 plants, though the term is sometimes used to include minerals, rocks, climate, 

 etc. 



Natural selection. The survival of the fittest individuals or the fittest species in a 

 variable population. 



Nauplius {naiv' pli ris). The earliest larval stage of shrimps, barnacles, and some 

 other Crustacea. 



Nautiloid. One of the extinct cephalopods resembling, Nautilus. 



Nautilus. An animal belonging to the Cephalopoda, living in a coiled shell divided 

 into chambers. 



Neanderthal man (no ahn' der tahl). A primitive man whose remains have been 

 found in various places in Europe. 



Necator. The genus of roundworms to which the hookworm belongs. 



Nectocalyx {nek' to ka' liks) {pi., nectocalyces, nek' to ka' li seez). One of the swim- 

 ming members of a siphonophore colony. 



Necturus. A genus of salamanders; the mud puppy. 



Nemathelminthes {nem' a thel min' theez). The phylum of roundworms and their 

 allies. For definition see Chap. 19. 



Nematocyst {nem' a to sisi). One of the stinging bodies of Hydra and other coelen- 

 terates. 



Nematode {nem' a tode). Any roundworm of the class Nematoda, phylum Nemathel- 

 minthes. 



Nematomorpha {nem' a to mar' fa). A group of wormlike animals of uncertain 

 affinities. They have usually been doubtfully included in the Nemathelminthes. 

 For definition see Chap. 19. 



Nemertinea {nem' er tin' e a). A group of wormlike animnls of uncertain relation- 

 ships. They are regarded by some as a class of Platyhelminthes. For definition 

 see Chap. 19. 



Nephridiopore {ne frid' i o pore). The external opening of a nephridium. 



Nephridium {ne frid' i um). An excretory organ of certain invertebrate animals 

 (worms, mollusks, etc.), approximately corresponding in function to the kidney 

 of vertebrates. It is commonly a coiled tube, as in the earthworm. 



Nephrostome (ne/' ro stome). The opening at the inner end of a nephridium as in 

 the earthworm. Also an opening (originally like that in the earthworm) con- 

 necting the coelom with the blood vessels of the kidney in certain Amphibia. 



Nereis, A genus of marine worms, phylum Annelida. 



Nerve. A bundle of axons or dendrites of nerve cells or of both axons and dendrites. 



