5 i6 



POLYZOA 



indiscriminate use of this method of arranging the Polyzoa. The old 

 genus Uschara, composed of forms with an erect coral-like habit, 1 

 included species which are now placed in such different genera as 

 Lepralia, Porella, Microporella, etc. The older works on Polyzoa 

 include all encrusting forms of Cheilostomata, with a completely 

 calcareous front wall, in the genus Lepralia, the members of which 

 are now distributed in numerous widely separated genera. 



As an instance of the converse arrangement essential sii 

 larity of the zooecia with great differences of the general habit- 

 may be mentioned the common Membranipora (Ulectra) pile 

 Ordinarily growing in the form of close encrustations on seaweeds, 

 this species may take on entirely different habits of growth. The 

 zooecia are now dissociated, growing in single lines over the sub- 

 stratum ; now forming erect tufts, composed of single lines of 

 zooecia or of several rows. The erect, branching habit appears 

 to be induced in the first instance by the character of the sea- 

 weed on which the colony begins life. Thus colonies which 

 encrust the thin branches of Corallina may have impressed on 

 them something of the mode of growth of the seaweed, so that 

 when they extend beyond the tips of the branches of the 

 Corallina, they continue to grow in delicate branches, which 

 still retain more or less the same diameter as those which 

 form their base. An extreme variation results in the beautiful 

 form known as Electra verticillata, in which the zooecia are arranged 

 with great regularity in whorls, which together form erect 

 branches. 3 But with all these variations, the zooecia are so much 

 alike that it is hardly possible to regard the extreme forms 

 as more than varieties of a single species. A careful examina- 

 tion of this case would convince most observers that the char- 

 acters of the zooecium are a more trustworthy guide to classifica- 

 tion than those of the entire colony, a result which was first 

 clearly stated by Smitt, and amply confirmed by Hincks. 4 



The avicularia of the Cheilostomata afford useful help in 

 classifying this group ; but while certain genera are always pro- 

 vided with avicularia, others include some species with these 

 organs, and other species without them. Again, while the species 



1 Cf. Milne-Edwards (H.), Ann. Sci. Nat. 2 ser. vi. 1836, pp. 5, 321. 



2 See Norman, Ann. Nat. Hist. ser. 6, xiii. 1894, p. 114. 



3 See Holdsworth, P. Zool. Soc. pt. xxvi. 1858, p. 306. 



4 Brit. Mar. Polyzoa, Introduction, p. exxii. 



