GYMNOBLASTEA 



i6i 



Among the hydroids with sessile medusoids or gonophores there 

 are many forms in which the medusoid structure is lost, and a bud- 

 like structure is found in which a transverse section shows simply 



plg.c. /per.c 



-c.end. 



Fig. 1 20. I. A, Eye oiLizzia koellikeri seen from the side, magnified, B, The 

 same seen from in front. C, Isolated cells of the same. From O. and R. Hert- 

 wig. Is. lens; per.c. percipient cells; pig.c. pigment cells. II. Radial section 

 through the edge of the umbrella of Carmarina hastata showing sense organ 

 and velum, a.n. auditory nerve; c.end. continuation of endoderm along 

 aboral surface; cm. circular muscles of velum; l.n.r. lower nerve ring; 

 msg. mesogloea ; ntc. nematocysts ; ra.v. radial canal running into circular 

 canal, both lined by endoderm; tct. sense organ or tentaculocyst ; u.n.r. 

 upper nerve ring; vm. velum. 



Fig. 121. Margellium, example of an anthomedusan. M. mouth; man. manu- 

 brium; t. testis; ten. or. oral and ten.m. marginal tentacles. Original. 



successive layers of ectoderm with generative cells, structureless 

 lamella and endoderm round an enteron which does not open by a 

 mouth. In forms like this the migrations of germ cells, mentioned as 



f%. 



