472 POLYZCU. 



globules, by the accumulation of which the polyp is ultimately formed. 

 The bud itself is hollow, and communicates with the parent stem : it 

 therefore has nothing in its composition resembling that of an egg; 

 neither distinct vesicle nor vitellus. The newly-formed cell soon grows 

 taller, and its lining membrane becomes thicker, indicating the first ap- 

 pearance of the intestinal canal, which is at first a simple cavity 

 bounded by the thickened lining of the cell. This cavity once formed, 

 the development of the different organs proceeds rapidly. First there 

 appears a longitudinal fold, resembling two lips, that, as they approach 

 each other, divide the cavity of the body into an anterior and posterior 

 compartment. The two lips, which have a valvular appearance, become 

 very regularly indented along their margins, and are soon recognizable 

 as the rudiments of the tentacular circle. 



(1232.) At this epoch, it must be remarked, the polyp presents two 

 cavities distinct from each other : there is a space between the walls of 

 the body and the parietes of the future alimentary canal, the interspace 

 being in communication with the stem of the parent polyp, and filled 

 with a fluid that is analogous to the blood of higher animals ; superiorly 

 this cavity likewise passes into the tentacles, and the fluid which bathes 

 the exterior of the alimentary canal thus finds admission even to the 

 extremities of those organs. The second cavity, which is the intestinal 

 canal, has as yet no communication with the external world. As the 

 formation of the tentacles proceeds, the portion which is situated in 

 front of them will become the sheath, and the other part the intestine. 

 As the tentacula are formed by the prolongation of the tubercles, which 

 were their first rudiments, the cavity of the stomach and the rest of the 

 intestinal tube gradually become apparent ; and at the same time some 

 globules are visibly disposed around the cut de sac of the former viscus, 

 which gradually become arranged into fibrillse, and constitute the re- 

 tractor muscles. 



(1233.) When the cell has nearly reached its full development, its 

 parietes become softened, and an opening is formed, which brings the 

 young polyp into communication with the surrounding element. The 

 Polyzoon has now attained its complete form, and can expand its ten- 

 tacula ; but, as yet, there are no traces of a generative apparatus, which 

 seems to be matured at a subsequent period. 



(1234.) Reproduction is likewise effected, in the Polyzoa, by means of 

 true ova. The ovary in which these are developed is situated immedi- 

 ately above the stomach, and is generally found containing eggs in 

 different stages of growth. In the same vicinity is situated another 

 viscus, regarded by Van Beneden as the testes, his opinion being founded 

 on the fact that, when a mature specimen of the animal is placed 

 between two plates of glass and gently compressed, so as to rupture its 

 parietes and cause the escape of the viscera, spermatozoa are easily 

 discoverable in its interior. 



