PLANESIS 



1110 



PLANKTON 



Ps. of the Pelvis, four planes are generally described : 

 the plane of the inlet, that of the outlet, the plane of 

 pelvic expansion, and that of pelvic contraction. The 

 plane of the inlet, or brim, is bordered by the linea 

 terminalis, and has an elliptic contour, with a depres- 

 sion posteriorly, produced by the projection of the sacral 

 promontory. Its diameters are — the anteroposterior, 

 or conjugate, from the upper edge of the promontory of 

 the sacrum to a point an eighth of an inch below the 

 upper border of the symphysis, measuring II cm. ; 

 the transverse, the longest possible transverse distance, 

 measuring 13^2 cm - > an d the oblique, from the upper 

 edge of one sacro-iliac junction to the opposite ilio- 

 pectineal eminence, measuring I2|^ cm. The plane 

 of the outlet is bounded by the sub-pubic ligament, 

 the pubic rami, the rami and tuberosities of the ischia, 

 the sciatic ligaments, and the coccyx. Its diameters 

 are — the antero-posterior, measuring 9^ cm (during 

 labor II cm.), and the transverse, measuring II cm. 

 The plane of pelvic expansion perforates the middle of 

 the symphysis, the tops of the acetabula, and the sac- 

 rum between the second and third vertebrae. Its dia- 

 meters are — the antero-posterior, measuring 12^ cm., 

 and the transverse, measuring 1 2^J cm. The plane of 

 pelvic contraction passes through the tip of the sac- 

 rum, the spines of the ischia, and the under surface of 

 the symphysis. Its diameters are — the antero-posterior, 

 measuring 1 1 ^ cm., and the transverse, measuring 

 \o)/ 2 cm. See Pelvis. P. of Refraction, a plane 

 passing through a refracted ray of light and drawn 

 perpendicular to the surface of the refracting medium. 

 P. of Regard, a plane cutting the center of rotation 

 of the eye and the point of fixation P. of Rolle, 

 in craniometry, that passing through the auricular and 

 the alveolar points. P., Sagittal, any antero-posterior 

 (cephalo-caudal) plane parallel with the meson or me- 

 dian plane. P., Single-inclined, a board arranged 

 as an inclined plane, with the distal end elevated; 

 it is used in the treatment of fractures of the lower 

 limb. P. -symmetric, in. chemistry, a term intro- 

 duced by Wislicenus to indicate that class of unsatu- 

 rated stereo- isomers in which similar substituting 

 radicles lie on the same side of a plane passing 

 through the points of junction of the linked carbon 

 atoms. The contrasting condition is central (or axial ) 

 symmetric in which similar radicles are symmetric 

 to an axis passing through the linked atoms. 

 Malelc and fumaric acids are respectively plane-sym- 

 metric, and central -symmetric stereo-isomers. P., 

 Van Shering's. See P. , Horizontal (of Van Shering). 

 P., Visual, the plane passing through the visual axis. 

 P., Visual (of Broca) , the orbital plane. 



Planesis (plan-e / -sis) [k '/Avqaiq, wandering]. Wander- 

 ing ; mental aberration ; metastasis. 



Planicaudate (plan-ik-aw' -daf) [planus, flat; cauda, 

 tail]. In biology, having a flattened tail. 



Planicipital ( plan-is-ip'-it-al) [planus, flat; caput, 

 head]. In biology, having a flat head. 



Planidorsate (pian-id-or'-sdl) [planus, flat; dorsum, 

 back]. In biology, having a flat back. 



Planiform (plan'-if-orm) [planus, flat ; forma, form]. 

 In anatomy, presenting a flat surface. 



Planipennate (plan-ip-en'-at) [planus, flat; penna, 

 wing]. In biology, having flat wings. 



Planipetalous (plan-ip-et'-al-us) [planus, flat; Trera- 

 Aov, petal]. In biology, having fiat petals. 



Planirostral (plan-e-ros'-tral) [planus, flat; roitrum, 

 beak]. In biology, having a flattened beak. 



Planispiral (plan-is-pi'-ral) [planus, flat; spira, a 

 coil]. In biology, coiled in one plane. 



Planities (plan 1 '-it-ez) [planus, flat]. Plane; also, 

 the sole of the foot. 



Planktology (plank-tol' -o-je) [plankton, the drifting 

 life of the sea, from n?\,ayxrog, wandering, roaming; 

 %6yoc, science]. In biology, the department of sci- 

 ence which treats of the constituents, charactei, dis 

 tribution, evolution and relations of the plankton or 

 drifting organisms of the ocean. 



Plankton (plank' -ton)[-'kayxT6c , wandering, roaming]. 

 In biology, the term proposed by Hensen and adopted 

 by Hseckel to designate all plants and animals found 

 at the surface of the ocean and which are carried about 

 involuntarily in the water (ploteric). It corresponds 

 to the. German terms Auftrieb and " pelagischer Mul- 

 der" introduced by Johannes Midler; and is employed 

 in contradistinction to benthos (fiivdoq, the bottom of 

 the ocean ; hence the organisms living there) and 

 nekton (the actively swimming surface organisms). 

 Plankton may be separated into Limnoplankton, the 

 swimming and floating population of fresh water, and 

 Haliplankton or that of salt water ; or again into 

 Oceanic Plankton or that of the open ocean, and 

 Neritic Plankton, or that of the coast regions. 

 Haeckel also distinguishes : (a) Pelagic Plankton, 

 those actively swimming or passively floating animals 

 or plants which are taken at the surface of the sea ; no 

 matter whether they are found alone or at a variable 

 depth below the surface, they constitute the super- 

 ficial and interzonary organisms of Chun. (b) 

 Zonary Plankton, those organisms which occur only 

 at definite depths of the ocean, and are only occasion- 

 ally found above and below this, e.g., many J 

 daria and Crustacea and Siphonophora. (c) Bathy- 

 bic Plankton, animals of the deep sea, which only 

 hover upon the bottom but never touch it, whether they 

 stand in definite relation to the abyssal benthos or not, 

 e. g., Tomapteris euchata, Megalocereus abyssorum. 

 (d) Autopelagic Plankton, the constant superficial 

 fauna and flora of the sea, e.g., Eucopidce, Porska- 

 HdcE, Eucharis, sp., Bolina, sp., Sagitta pur. 

 Pontellina, sp., etc. (e) Bathypelagic Plankton, 

 all those organisms which occur not merely at thi 

 face, but extend down into the depths ; these are 

 called by Chun interzonary pelagic animals. Here 

 belongs properly the chief mass of the plankton. 

 These last organisms may be distinguished as 

 Nyctipelagic , those which rise to the surface onh 

 at night, living in the depths during the day ; very 

 many Medusa, Siphonophora, Pyrosoma, most Pttro 

 poda and Hcteropoda , very many Crustacea, etc j 

 Chimopelagic, those which appear at the surface only 

 in winter, and in summer are hidden in the depths ; 

 Radiolaria, Medusa, Siphonophora, Ctcnophc 

 part of the Pteropoda, and Hcteropoda, many Crusta- 

 cea, etc. (3) Allopelagic, those which perform in 

 lar vertical wanderings, sometimes appearing at th 

 surface, sometimes in the depths, independently oi 

 changes of temperature. (/) Spanipelagic Plank- l 

 ton, those animals which always live in 1 

 depths (zonary or bathybic) and come to tl 

 only exceptionally and rarely. Holoplanktonic t 

 ganisms, those that pass their whole life and 

 cycle of development hovering in the ocean, • 

 no relation whatever to the benthos; hen 

 the greater part of the diatoms, and oscillarit, all 

 Murraycites and Peridinea, all Radiolaria, n 

 Globigerina, the hypogenetic Medusa, all Sif>m 

 phora and Ctenophora, all Clnctognatha, 

 the Copelata, Pyrosoma, and Thalidia, etc. Mero 

 planktonic organisms, on the contrary, are found 

 the sea only for a part of their life, passing the 

 part vagrant or sessile in the benthos, 

 the diatoms and oscillaria, the planktonic fu 

 metagenetic medusre, some turbellarians and ann< 



