1S8 



IX^EIITF.BIIATI-: AXnrALS. 



each consisting iif a numbev of distinct but similar zouids, produced 

 by gemmation from a single primordial individual. The colonies 

 thus i>roduceil are very generally j)rotected l.)y a horny or chitinous 

 integument, and tliey are so like the Hydroid Zoophj-tes that they 

 were long described as sucli. Indeed, the only absolute distinction 

 between the two classes is to be fo\ind in the internal structure of 

 the zoijids of each. The separate beings or zooids which collec- 

 tively constitute the colony of any J'nji/:i}iiii are spoken of as " poly- 

 pides " — the ienn po/>/pi/i/ lieing only used in connection with the 

 Jfi/drnzo'i, and the ternj /ic/i/j)' being similarly restricted to the 

 Artiiioznii. 



Each jiolypide in a ty|iical J'(i/i/:i«in has the following structure 

 (fig. 132, 2) : The body of tlie animal is enclosed in a double-walled 

 sac, of which the outer layer is usually chitinous or calcareous, and 

 constitutes a "cell" in wliicli the ZDiiid is contained. This outer 

 layer is known as the "ectucyst" to distingui.sli it from the ecto- 



Fi;,'. 132.— Mcinili.iloKy uf Polyzua. ]. Friigmciit of one of tlic Swi-iiiats (Fhisim 



!, >uuah') mir^n'iWrd to show II lis. 2. Diiigiain of a siiigk- liolypiile of a Poly- 



z.iMii (afl.-r j\llHi,'iii): « .Mmilli sinToiiiHlecl liy flip ciliate.l tentacles; b Alimentary 

 e.in.'cl ; r Anus; ./ .\ervi-:.;,in:4linii , , hi vi'sl hi- sac "1- " eetocyst " ; // Repi-. dlie- 

 tive oi-inis; (/ Jliisrle. ::. liinrs-lie.'i.l ]iniei>ss. 



derm of the ('leleiitefiifii. The cell, thus fc.rmed, is lined by a much 

 more delicate membranous layer, wliii'li is known as the " endocyst." 

 'J'his niembiaiious ,sac, fin'mcd liy the endocyst, is jjierced by two 

 openings. (.)iie of tlii'se is the motitli, and it is alwtiys surrounded 

 bv a rii-ile (,!■ i-ii'si-cnl (jf liolliiw rili.ateil |>rocesses or tentacles (lig. 



