136 



ZOOLOGY 



SECT. 



precisely the same layers as that of the hydranth — ectoderm, 

 raesogloea, and endoderm. The ectoderm is continued on to the 

 sub-umbrella, and then round the margin of the bell on to the 

 ex-umbrella, so that both surfaces of the bell are covered with 

 ectoderm. The endoderm is continued from the base of the enteric 

 cavity into the radial canals and thence to the circular canal, 

 so that the whole canal-system is lined by endoderm. In the 

 portions of the bell between the radial canals there is found, 

 between the outer and inner layers of ectoderm, a thin sheet of 

 endoderm, the cndoderm-Iamdla {end. lam), which stretches 

 between adjacent radial canals and between the circular canal 

 and the enteric cavity. In the bell, as in the manubrium, a 



Fig. 100 — Dissection of a medusa with rathennore tliau one-quarter of the umbrella and manu- 

 brium cut away (diagrammatic). The ectoderm is dotted, the endoderm striated, and the 

 mesoglcea black, circ. c. cii'cular canal ; end. lam. endoderm lamella ; gon. gonad ; l. lithocyst ; 

 mnb. manubrium ; mth. mouth ; rad. c. radial canal ; vl. velum. 



layer of mesoglcea everywhere intervenes between ectoderm and 

 endoderm. 



The velum (vl) consists of a double layer of ectoderm and a 

 middle one of mesoglcea : there is no extension of endoderm into 

 it. The tentacles, like those of the hydranth, are formed of a 

 core of endoderm covered by ectoderm, the cells of the latter 

 being abundantly supplied with stinging-capsules. 



Comparison of Polype and Medusa. — Striking as is the 

 difference between a polype and a medusa, they are strictly 

 homologous structures, and the more complex medusa is readily 

 derivable from the simpler polype-form. It is obvious, in the 

 first instance, that the apex of the umbrella corresponds with the 

 base of a hydranth (Fig. 101, A and D), being the part by which 

 the zooid is attached in each case to the parent stem : the mouth 

 with the manubrium are also obviously homologous structui-es in 



