50 



is just the condition which exists in the specimen sho^vn in 

 figure 26, The mimbers indicating the s ense-oi'cins are put 

 at the intex-vals between t'-iose indicating tentacles, to 

 show that while the eight tentacles between the eight-tenta- 

 cled and the 16 tentacled stages, for instance, are appear- 

 ing, the four sense-organs which make up the second quad- 

 rant are appearing. 



Another rule is followed by the marginal organs in 

 their order of apjjearance; P'ach new tentacle arises not on- 

 ly just in front of a sense-organ, but in a definite loca- 

 tion with reference to the tentacles already present. And 

 the same thing is ti-ue for the sense-organs. It is there- 

 fore possible, from a study of successive stages, to pre- 

 dict where each new quartette of tentacles or sense-organs 

 will arise. The diagrams shown in text-figures 3 to 8 

 are from camera drawings of mounted whole specimens of me- 

 dusae. If we examine text-figure 4 v/o see that T (tenta- 

 cle) III follows T I; and text-figure b shows TIV following 

 T II. Thus T III and T IV form a series, arising in corres- 

 pending positions in the (juadrant relative to the tentacles 

 already present. (By "series" is not meant "cycle", with 

 the idea of simultaneous appearance; tlie notion is rather 



