ISO ZOOLOGY. 



with the Polyzoa, Bracliiopods, and possibly the Mollusca, 

 the latter branch being probably a modified vermian type, 

 and with an ancestry not unlike that of the Rotifers and 

 aberrant, generalized Polyzoa and Bracliiopoda. The classi- 

 fication of the Rotatoria is in an unsettled state, the group 

 probably consisting of three orders, viz. : the true Rotatoria, 

 the Echinoderidce, and Gastrotricha, 



CLASS III. -ROTATORIA. 



Worms with usually more or less solid segments, very unequally developed, 

 bearing a ciliated velum, the mouth opening into a mastax ; sexes separate, 

 the males much smaller, more rudimentary than the females. A small 

 nertoua ganglion. No circulatory apparatus, but with a voluminous excre- 

 tory (water-vascular) organ. 



(Albcrtia, Asplauchna, Ilydatiua, Bracliioiius, Rotifer, aud the 

 highest form, Floscularia.) 



Laboratory Work. The Rotifers can only be studied while alive and 

 as transparent objects. Little is known about the American species. 



CLASS IV. POLYZOA (Moss Animals). 



The Polyzoa, though not commonly met with in fresh 

 water, are among the commonest objects of the seashore. 

 They are minute, almost microscopic creatures, social, grow- 

 ing in communities of cells (called poly- 

 zoaria or corms), forming patches on sea- 

 weeds and stones (Fig. 124, Memlranipora 

 solida Pack.). Certain deep-water species 

 grow in coral-like forms (Fig. 125, Myrio- 

 zoum subgracile D'Orbigny), while the 

 chitiuous or horny Polyzoa (Fig. 126, 

 HalopMla lorealis Pack.), are often mis- 

 Fig. 124. ceils of Sea- taken for sea- weeds on the one hand, and 

 Sertularian Hydroids on the other. From 

 their likeness to mosses the name Bryozoa was given to the 

 group by Ehrenberg, a year after Thompson (1830) had 

 called them Polyzoa, so that the latter name has priority. 



