188 



ZOOLOGY 



SECT. 



the mesenteries and of their muscles is very definite and charac- 

 teristic (Fig. 138, B). At each end of the gullet, opposite the 

 siphonoglyphe, are two mesenteries (d. mes.), having their longi- 

 tudinal muscles turned away from one another : they are distin- 

 guished as the directive mesenteries, and, in the case of Tealia, 

 there are two couples of directive mesenteries, one at each end of 

 the long axis of the gullet. Of the remaining complete or 

 primary mesenteries there are four couples on each side (mes. 1), 

 differing from the directive couples in having the longitudinal 

 muscles turned towards one another. The secondary and tertiary 



ntc 



end 



FIQ. 140. Tealia crassicornis. Trans- 

 verse section of tentacle, ect. ectoderm ; 

 end. endoderm ; l.m. longitudinal muscles ; 

 msffl. mesogloea ; nv. c. nerve-cells ; nv. f. 

 nerve-fibres ; ntc. nematocysts ; t. m. 

 transverse muscles. (After Hertwig.) 



FIG. 141 . Three nematocysts of 

 Sagartia. (After Hertwig.) 



mesenteries (mes. 2, mes. 3) are also arranged in couples, and in all 

 of them the longitudinal muscles of each couple face one another. 



Symmetry. It will be noticed that Tealia, unlike the typical 

 hydrozoan and scyphozoan polypes, presents a distinct bilateral sym- 

 metry, underlying, as it were, its superficial radial symmetry. It 

 is divisible into equal and similar halves by two planes only, viz., a 

 vertical or sagittal plane taken through the long diameter of the gullet, 

 and a transverse plane taken through its short diameter. 



The general microscopic structure of a Sea-anemone is well 

 shown by a section through a tentacle (Fig. 140). Both ectoderm 

 (ect.) and endoderm (end.) consist mainly of very long columnar, 



