i6o 



THE INVERTEBRATA 



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, the mouth never opens 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 in groups round the margin of the bell, 

 and four double tentacles at the end of the manubrium. Lizzia 



can.c. 



end. la 



--ten.on 



Fig. 119. Longitudinal sections through gonophores of TM^M/ana. A, Young 

 male. B, Female with larvae. In B the details of tissues are omitted ; in A 

 ectoderm is black, endoderm cross-hatched, end. lam. endoderm lamella; 

 act. actinula larva ; pla. planula larva with rudiments of aboral tentacles ; 

 mb. manubrium; ov. ovum; stk. stalk of polyp. Other letters as in Fig. 117. 

 Original. 



possesses eight "eyes" (Fig. 120 1) which are little patches of ecto- 

 derm, 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. 121) has no eyes but apparently 

 suffers no disability from their absence : probably the light-perceiving 

 cells are scattered over the general surface of the ectoderm. "Eyes" 

 are however a general character of the Anthomedusa as "Ears" (as 

 statocysts may be broadly termed) are of the Leptomedusa. 



