206 



ZOOLOGY 



SECT. 



as to line its ventral portion : in this way the stomodseum (st.) is 

 produced. The remainder of the cavity widens out and becomes 



me 



me 



FIG. 153. Three stages iu the development of Callianira. -'. iufundilmlum ; co. ectodenn ; 

 i ,'. endoderm ; me. mesoderm ; st. stomodseum. (From Lang's Comparativi Anatomy.) 



the definitive infundibulum (d.), and before long sends off four 

 adradial pouches, the rudiments of the canal-system. At the same 

 time a gelatinous layer (Fig. 154, g.), the mesoglcea, makes its 



appearance between the ectoderm and 

 endoderm. 



The later processes of development 

 may be described very briefly. The 

 canal-system gradually assumes its 

 adult complexity and the swimming- 

 plates appear. A thickening of the 

 ectoderm on each side of the body 

 gives rise to the epithelium of the 

 tentacle and of its pouch. The muscle- 

 fibres forming the axis of the tentacle 

 (B, me.) are derived from the mesoderm, 

 which also gives rise to the contractile 

 fibres of the mesoglcea (me 1 .). The 

 otoliths are formed in the ectoderm- 

 cells of the apical pole, but gradually 

 make their way on to the free surface 

 of the cells and become supported on 

 four groups of fused cilia. Four outer 

 groups of cilia unite with one another 

 to form the bell (.$/,'.). 



The most noteworthy points in this 

 somewhat complex process of develop- 

 ment are the following : 



1. The distinction between a purely 

 protoplasmic part of the egg and a yolk- 

 ^etop^nt^lsSa^ira! containing portion. In the Hydrozoa 



d.infundibulum ; c. endoderm ; anc { Actinozoa the Volk-material is 

 a. mesoglcea; /' . mesoderm; ... , ^ , -.. .-. -, 



si-, st-nse-organ ; st. st,,m,.d;<-uui ; small in amount ana evenly distributed. 

 te "^;X;o La " g ' ! the egg being described as alecithal or 



