GYMNOBLASTEA 



139 



which is really a polyp of Tuhularia with a short stem. At this stage 

 it makes its way out of the shelter of the gonophore and fixes by its 

 aboral end. As a rule, only one of these large eggs can be produced 

 at once and a ripe gonophore generally contains two larvae of different 

 ages, one a planula and the other an actinula, which may be seen 

 protruding from the aperture of the bell. 



In such gonophores the neuromuscular structures of the bell are 

 hardly developed at all and there are no evident sense organs. In the 

 medusae called Lizzia and Margellium, common plankton forms 

 whose polyp stages are not known, we see the normal anthomedusan 

 type. In both of these there are a number of short tentacles, arranged 



ntc. 



c.end. 



l.n.r. 

 II. a.h. 



Fig. 117. I. A, Eye of Lizzia 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; /.w.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. 



in groups round the margin of the bell, and four double tentacles at 

 the end of the manubrium. Liszia possesses eight "eyes" (Fig. 

 117 I) which are little patches of ectoderm, in which some of the 

 cells develop pigment while others elongate and end in rods. The 

 latter are concluded to be the light-perceiving cells. There is also an 

 outer enlargement of the cuticle which serves to concentrate light on 

 the organ and may be called a lens. Though there is no direct evidence 

 that these organs have a relation to light, they have in a simple form 

 all the structural elements of the eye of higher animals. Margellium 

 (Fig. 118) has no eyes but apparently suffers no disability from 

 their absence : probably the light-perceiving cells are scattered over 



