NOTEUS. 



[ 465 



NUCLEUS. 



involved in a matrix of mucus, social, swim- 

 ming in masses. Marine. 



[Om- knowledge of these plants is very 

 unsatisfactory. Kiitzing enumerates a vast 

 number of species, which would be probably 

 reduced to a comparatively small number if 

 the history of development were known. 

 We cannot see any distinctive character in 

 his genus Hormosiphon; the filaments of 

 Nostoc have proper sheaths when first deve- 

 loped. The natiu-e and functions of the so- 

 called spermatic and vesicular cells require 

 further elucidation.] 



BiBL, Ralfs, on NostochinecB, Annals of 

 Nat. Hist. 2 ser. vol. v. 321. pis. 8 & 9; 

 Kiitzing, Tabulce Phycologiccs, B. i. p. .91- 

 100. Bd. ii. pi. 1-15 ; Thuret, on Nostoc 

 verrucosum, Ann. des Sc. nat. 2 ser. t. ii. ; 

 Meneghini, Monographia Nostochinearum 

 italicarum, Mem. Turin Acad. ser. 2. v. 1843; 

 AUman, Micr. Journal, 1855. 



NOTEUS, Ehr.— A genus of Rotatoria, 

 of the family Brachionaia. 



Char. Eyes absent; foot forked (=: eye- 

 less Brachionus). 



N. quadricornis (PI. 35. fig. 13). Cara- 

 pace suborbicular, depressed, scabrous, areo- 

 late, with fom' spines in front, and two be- 

 hind; aquatic; length 1-120 to 1-/0". 

 BiBL. Ehrenberg, Infus. p. 502. 

 NOTODELPHYS, Allm.— A genus of 

 Entomostraca, of the order Copepoda. 



N. ascidicola (PI. 14. fig. 22) resembles 

 Cyclops in general appearance. The external 

 ovary is a single organ, lying across the back 

 of the abdomen ; eye single. Marine. 



BiBL. Allman, Ann. Nat. Hist. xx. p. 1 ; 

 Baird, Brit. Entom. p. 237- 



NOTOMMATA, Ehr.— A genus of Rota- 

 toria, of the family Hydatinsea. 



Char. Free ; eye single, cervical ; tail-like 

 foot with two toes ; rotatory organ simply 

 ciliated. 



In some the rotatory organ is extended 

 laterally in an ear- or arm-like form. 



Ehrenberg describes twenty-three species, 

 some of which are parasitic, N. petromyzon 

 and parasita living within Volvox glohator, 

 and N, Werneckii within the vesicles of 

 Vaucheria ; and divides them into the sub- 

 genera : Labidodon, jaws each with a single 

 tooth; Ctenodon, jaws each with several 

 teeth. 



Many of the species are large and well 

 adapted for the study of the internal struc- 

 ture. 



M. centrura (PI. 35. figs. 14 ; 15, jaws and 

 teeth). Body attenuate at each end, foot 



small and hard ; cephalic auricles short ; no 

 lateral setae ; aquatic ; length 1-36". 



BiBL. Ehrenberg, /w/m5. p. 424 ; Dujar- 

 din, Infus. p. 646. 



NOTONECTA, L.— A genus of aquatic 

 Hemipterous insects. 



N. glauca is common in pools. Its setse 

 or lancets, and natatorial hind -legs form in- 

 teresting microscopic objects. 



Fig. 535, 



Notonecta glauca. 

 Magnified 3 diameters. 



NUCLEUS and NUCLEOLUS of 

 Plants. — The term nucleus is applied in 

 botany to two very different things ; first to 

 the central body of the young ovules of 

 Flowering plants, and secondly to a peculiar 

 structure met with in the interior of cells. 

 The first will be described under the head of 

 Ovule; the cell-nucleus and nucleolus, 

 mentioned in the article Ce-li, {Vegetable), 

 will be discussed here. 



Few parts of the minute organization of 

 plants are more obscure than the structure 

 and function of nuclei ; some authors regard 

 them as of the highest physiological import- 

 ance, others consider their import altogether 

 unkno\Mi. The nucleus may be observed 

 most easily in the parenchymatous cells of 

 the herbaceous structures and flowers of 

 Monocotyledons (PI. 36. fig. 28 b), or in the 

 young cells of the hairs of Flowering plants 

 generally (PL 38. figs. 8, 9 b), or in the em- 

 bryo-sacs of unfertilized ovules (PI. 38. figs. 

 4-6) ; in such cases the characters are well 

 defined and unmistakeable. It consists of a 

 lenticular body formed of more or less gra- 

 nular substance, apparently not diverse from 

 the Protoplasm, with one or more well- 

 or ill-defined bright points or cavities {nu- 

 cleoli) in the interior. Wherever it appears 

 throughout the higher plants, it seems to 

 possess the same characters. Nageli indeed 

 declares that it is a vesicle ; but we believe 

 this to be an error, that real nuclei are ordi- 

 narily solid, although bodies akin to them, 



2h 



