POLYGASTRICA. 



[ 522 ] 



POLYPI. 



branch at their lower ends, and each produce 

 four rudimentary embryos; all but one of 

 them vanishing during the ripening of the 

 seeds. Our space only admits of a brief 

 notice of these interesting phenomena, on 

 which much interesting information will be 

 found in the works referred to below. 



BIBL. Meyen, On Impregnation and Poly- 

 embryony (Berlin, 1840), transl. in Taylor's 

 Scientific Memoirs, iii. p. 1 ; R. Brown, Ann. 

 Nat. Hist. xiii. p. 368; Mirbel and Spach, 

 Ann. des Sc. nat. 2 ser. xx. p. 257 ; Criiger, 

 Botanisch. Zeit. ix. p. 57 ; Gelesnoif, Ann. 

 des Sc. nat. 3 ser. xiv. p. 189, and the works 

 of Hofmeister cited under OVULE. 



POLYGASTRICA. According to Eh- 

 renberg's system, the Infusoria are subdi- 

 vided into the Polygastrica and the Rota- 

 toria. The so-called Polygastrica correspond 

 to our Infusoria; the Rotatoria form a 

 distinct class. 



POLYIDES, Ag. A genus of Crypto- 

 nemiacese (Florideous Algae), containing one 

 British species, P. rotundus, having a 

 branched frond 4 to 6" high, consisting of 

 repeatedly dichotomous, purplish-brown, 

 solid fibres, about 1-20" in diameter. The 

 fibres present a central layer of longitudi- 

 nally arranged filamentous cells, and a cor- 

 tical layer of perpendicular, dichotomous 

 filaments, formed of elliptical cells internally, 

 terminating at the surface in minute nionili- 

 form rows. The fructification consists 1 . of 

 favellce bearing spores, contained in super- 

 ficial wart-like bodies, composed of colourless 

 articulate filaments ; 2. tetrahedrally divided 

 tetraspores, imbedded in the peripheral fila- 

 ments of the cortical layer of the frond. 

 Antheridia have not yet been observed. 



BIBL. Harvey, Brit. Mar. Ala. p. 146. 

 pi. 18 D; Phyc.Brit. pi. 95; Greville, Ala. 

 Brit. pi. 11. 



POLYOMMATUS, Latr. A genus of 

 Lepidopterous Insects, of the family Lycae- 

 nidse. 



Char. Antennae terminated by a contracted 

 knob; tarsal claws minute; wings not tailed. 



The (thirteen) species are small butterflies, 

 the upper surface of the wings of a beautiful 

 blue colour, the under side gray or brownish, 

 and with numerous eye-like spots. 



The scales upon the under surface of the 

 wings of P. argiolus and P. argus have 

 been proposed as test-objects. They are of 

 two kinds, one resembling in structure the 

 ordinary scales of insects; the other of a 

 battledore form (PI. 27. figs. 20 & 21). See 

 SCALES OF INSECTS and TEST-OBJECTS. 



The species are figured in Westwood's 

 British Butterflies. 



POLYPHEMUS, Mull. A genus of En- 

 tomostraca, of the order Cladocera, and 

 family Polyphemidse. 



Char. Head distinct from the body ; ab- 

 domen long, slender and projecting exter- 

 nally from the shell. 



P. pediculus (PL 14. fig. 29). The only 

 species. Aquatic. 



BIBL. Baird, Brit. Entomostr. p. 111. 



POLYPI (Zoophytes). A class of the 

 Animal Kingdom. 



Char. Body rounded or elongate, with a 

 distinct mouth, surrounded by retractile ten- 

 tacles or radiating lobes ; individuals usually 

 aggregate, and furnished with a horny or 

 calcareous external or internal skeleton or 

 polypidom ; gemmiparous and oviparous. 



The polypes are usually enveloped in an 

 external (PL 33. figs. 4 6, 18 & 30), or sup- 

 ported by an internal axial skeleton (PI. 33. 

 fig. 6), called the polyparium or polypidom. 

 This is either horny, leathery or calcareous. 

 Most polypes are united into smaller or 

 larger groups by the polypidom, which often 

 possesses an elegant plant-like form. The 

 tubular or bell-shaped processes or cavities, 

 in which the body of the individual polypes 

 is contained, form the polype-cells : they 

 are sometimes furnished with a kind of lid ; 

 in some of the softer polypidoms a number 

 of spicula are present. 



The structure of the calcareous polypi- 

 doms has not been satisfactorily determined. 

 They are usually traversed by vascular canals, 

 and appear in some cases at least to consist 

 of spicula aggregated and fused together. 



The polypes are rarely free, or capable of 

 fixing themselves by a disk at the base of 

 the body, being usually fixed at the bottom 

 of the polype-cells of the polypidoms ; the 

 polypidoms being attached by a rooting base 

 to some foreign body. A distinct transparent 

 integument exists, which is frequently 

 covered by ciliated epithelium, especially 

 upon the tentacles. Imbedded in this are 

 stinging organs (PI. 33. fig. 22), resembling 

 in general those of the Acalephae. 



In many, distinct muscles are present; 

 but the fibres are not striated, although fre- 

 quently exhibiting transverse wrinkles. In 

 some polypes the substance of the body 

 consists entirely of sarcodic substance. In 

 many, both the integument when present, 

 and substance of the body contain scattered 

 calcareous spicula (PL 33. figs. 7, 27 & 28). 



Curious appendages called birds'- head- 



