58 



INVEETEBEATA 



CHAP. 



The mass is termed by German authors " Glockenkern," which 

 we may translate " bell-rudiment " ; it is crescentic in section with 

 its concavity directed downwards. Into this concavity fits a pro- 

 trusion of the endoderm of the medusa-bud ; this is the " spadix," 

 the rudiment of the future mauubrium, so far as its endodermal portion 

 is concerned. From the base of the spadix four hollow protrusions 



of endoderm grow out as canals, and 

 insinuate themselves between the ecto- 

 derm of the bud and the outer ectoderm 

 of the bell-rudiment. The spots where 

 these canals bud out are the interspaces 

 between four vertical, solid ridges of 

 endoderm called the taeniolae, which 

 project into the gastric cavity of the 

 spadix. 



The canals are termed the radial 

 canals, and they eventually push out 

 short protrusions of the ectoderm which 

 are the rudiments of the medusa 

 tentacles. The radial canals flatten 

 out and become fringed with flat 

 extensions of endoderm. These exten- 

 sions meet one another in the centre 

 of the interradii, and so constitute 

 continuous sheets of endoderm covering 

 the interradii and forming the so-called 

 endoderm lamella (Fig. 35, en.l}. 



The apex of the medusa-bud consists 

 only of the ectoderm of the bud and 

 the outer ectoderm of the bell-rudiment 

 through developing medusa-bud c i ose i y pres sed together. Absorption 



ot lubidana mesembriianthemwn. n , n T y i 



(After Gutte.) * these adpressed layers now takes 



P laC6 > and an Opening is thus formed 



which places the cavity ot the bell in 

 connection with the exterior, and 

 through this aperture the manubrium 

 often protrudes. The thin ectoderm 

 round the edge of the aperture forms 

 the velum. In Tubularia the hollow 

 radial canals are transitory structures, and their lumina soon become 

 crossed by cords of cells. 



The genital cells are found in the ectoderm covering the lower 

 part of the spadix. According to Brauer (1891) they originate as 

 interstitial cells of the ectoderm in the early stages of development 

 of the medusa-bud ; they then migrate through the jelly into the 

 endoderm, and finally iuto the ectoderm covering the spadix 

 (Fig. 36). 



FIG. 34. Three longitudinal sections 



In A, the youngest, the bell-rudiment 

 is just formed. In B, the spadix and 

 radial canals are differentiated. In C, the 

 umbrella-cavity is open to the exterior. 

 <//., bell-rudiment (Glockenkern); </'''''> 

 genital ectoderm ; r.c, radial canal ; up, 

 spadix ; t, taeniola ; u, umbrella-cavity ; 

 v, velum. 



