IV 



PHYLUM CCELENTERATA 



137 



end font 



ft.C 



'/re 



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

 endoderm, the endoderm-lamella (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 layer of 

 mesoglcea everywhere intervenes between ectoderm and endoderm. 



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

 middle one of mesogloea : 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 

 homol o g o u s 

 s t r u c t u res, 

 and the more 

 complex me- 

 dusa is readily 

 derivable from 

 the simpler 

 polype - form. 

 It is obvious, 

 in the first 

 instance, that 

 the apex of 

 the umbrella 

 c o r r e sponds 

 with the base 

 of a hydranth 

 (Fig. 102, 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 structures in 

 the two cases. Suppose the tentacular region of a polype to be 

 pulled out, as it were, into a disc-like form (B), and afterwards to 

 be bent into the form of a saucer (C) with the concavity distal, 

 i.e. towards the manubrium. The result of this would be a medusa- 

 like body (C, C') with a double wall to the entire bell, the narrow 

 space between the two layers containing a prolongation of the 

 enteron (ent. cav') and being lined with endoderm. From such a 

 form the actual condition of things found in the medusa would be 

 produced by the continuous cavity in the bell being for the most 

 part obliterated by the growing together of its walls so as to form 

 the endoderm-lamella (Z)', end. lam), and remaining only along 

 four meridional areas the radial canals (rod. c), and a circular 

 area close to the edge of the bell the circular canal (cir. c). 



Fio. 101. Dissection of a medusa with rather more than one-quarter 

 of the umbrella and manubrium cut away (diagrammatic). The 

 ectoderm is dotted, the endoderm striated, and the mesoglcea black ; 

 circ. c. circular canal ; end. lam. endoderm lamella ; gon. gonad ; /. 

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

 velum. 



