138 Papers from the Marine Biological Laboratory at Tortugas. 



In this examination, in 66 per cent of the plutei studied, arms with 

 latticed structure were found; no perfect Toxopneiistes plutei, and s per- 

 fect Hipponoe plutei were seen. 



In the study of the same cross made in 1910 there were 50 plutei 

 examined, 48 per cent of which had the latticed structure and 50 per 

 cent basket structure. 



ToxoPNEUSTES ? X Mellitac?: 



The Toxopneustes $ X Mellita d^ cross was made after the eggs 

 had stood in sea-water for 5 hours. A fertilization membrane was 

 formed. The fertilization-cleavage period was characteristic of the 

 egg. Most of the larvae obtained were of the mixed type. In general 

 the hybrids were of the maternal type in body form, but without excep- 

 tion had multiple rods in the anal arms. A few larvae, such as those 

 shown in plate 2, fig. 21, resemble the plutei of the pure maternal form, 

 although the hybrid origin was very evident. The body skeleton was 

 of the Mellita type, but without the posterior basket, and a very abnormal 

 fenestration of the anal arm skeleton was evident. Some of the Toxop- 

 neustes 9 X Mellita cP hybrids were kept alive for upwards of 10 days. 



Toxopneustes 9 X Moirac^: 



This cross was readily made after the Toxopneustes eggs had stood 

 in water for 5 hours, experiments showing that the best results were 



Fig. 5. — Pluteus Moirac^ X Toxopneustes 9. 



obtained after the eggs were allowed to stand for this length of time. 

 A fertilization membrane was formed. Cleavage took place in about 40 

 minutes after fertilization; the fertilization-cleavage period being that 

 characteristic of the egg. 



