512 



THE MICROSCOPE. 



The Polyzoa were placed by Dr. Johnston tinder 

 the head Ascidioida; in the generality of works they are 

 named Bryozoa, and the individual, Bryozoon ; derived 

 from the Greek words fipvov, sea- 

 moss; oon/, animal. (Fig. 243.) 

 The Polyzoa are all compound as- 

 sociated animals, whence their 

 name ; but when a polyzoon egg is 

 hatched, as in the case of Pluma- 

 tella, it commences life as an iso- 

 lated being, and by subsequent 

 growth, resembling budding, mul- 

 tiplies into a colony. All are most 

 bountifully supplied with cilia, and 

 the play of these is most energetic, 

 for the purpose of securing an 

 abundant supply of food, and ap- 

 parently without exertion on the 

 part of the creature itself. From 

 this most marked characteristic, 

 Dr. Farre was induced to give them 

 the name of Ciliobrachiata. But 

 it has at length been determined to 



Fig. m.-Bryozoon Bower- transfer the Polyzoa, f lustra, Le- 

 bankia, " Bowertank Bry- p ra iia Anguinaria spatulatO) &c. 



ozoon," showing its inter- * .. , y ., i j i 



__._ _ -!..._ to the sub-molluscan kingdom. 1 



Polyzoa are generally found living 

 together in great numbers, resem- 

 bling in this respect some of the Actinozoa, and are 

 protected by membranaceous coverings or polypidoms. 

 Protrusion and retraction are performed by two sets of 

 muscles, one acting on the body of the animal, the other 



(1) Mr.^Gosse, in his Manual of Marine Zoology, adopts the idea, now pretty 

 general, that the Polyzoa belong to the Molluscous division, in spite of their 

 external resemblances to Polypes, and he places them among Molluscs. In this, 

 perhaps, he has thought more of systematic views on classification than of the 

 student's convenience. It seems to us quite clear that, without adopting De 

 Blainville's principle of classifying animals according to their envelope as the 

 best principle of scientific classification, we should adopt it in works of refer- 

 ence like the present, since the external characters are necessarily those most 

 immediately recognised by the student ; and in the case of the Polyzoa, they are 

 so remarkably similar in external characteristics to the hydroid polypes, that 

 they were always classed with them, until the profounder investigations of Van 

 Beneden, Allman, and others, revealed the resemblances between the internal 

 characteristics of the Polyzoa and those of Molluscs. 



nal ' structure ; another 

 animal withdrawn mto its 

 cell. 



