DEPARTMENT OF MARINE BIOLOGY. 



137 



In Toxopneustes the skeleton of the anal arms consists of a single straight 

 rod with numerous thorn-like protuberances or "prickles." In Hipponoe the 

 skeleton of these arms is composed of three rods, connected by cross-bars, 

 to form a ladder-like or latticed structure. In these respects the forms in 

 consideration correspond respectively to the plutei of Strongylocentrotus 

 lividus and Sphcer echinus granulans, upon which the greater part of the- 

 work done by the European investigators has been based. 



In my study of the hybrids I have taken the presence of latticed skeletons 

 as an indication of Hipponoe dominance and the occurrence of more than 

 one rod in the anal arms, a most infrequent variation in pure Toxopneustes 

 plutei, as an indication of Hipponoe influence. 



The results of the investigation are most readily given in tabular form. A 

 few words of explanation suffice to make the meaning of the headings of the 

 columns clear. 



"Plutei with lattice structure" are plutei which have parallel rods, con- 

 nected by crossbars, in one or both anal arms. 



"Anal-arm rods with lattice structure" designates the total number of anal 

 arms of the plutei considered in the preceding column, which have a skele- 

 ton composed of parallel rods connected by crossbars. 



"Arms more than one rod" indicates an anal-arm skeleton of more than 

 one straight rod. 



"Perfect Hipponoe rods" are anal-arm rods which are as perfect as those 

 found in purely bred Hipponoe plutei. 



"Perfect Toxopneustes rods" indicates a single straight rod with prickles. 



"Perfect Toxopneustes plutei" are plutei of the normal Toxopneustes type^ 



"Perfect Hipponoe plutei" are plutei of the normal Hipponoe type. 



Summary of Results of Cross-FeHilizaiion. 



In ordinary sea-water. 



(Number of plutei studied, 50 ; temperature of 

 water, 28.5° C.) 



Hipponoe (f X Toxopneustes 9 



Toxopneustes d X Hipponoe ? 



In sea-waier of increased alkalinity. 



(400 c. cm. sea-water -f- 10 drops N/io NaOH ; 

 number of plutei studied, 50 ; temperature of 

 water, 29° C.) 



Hipponoe <f X Toxopneustes 9 



Toxopneustes cf X Hipponoe 9 



In sea-water 0/ decreased alkalinity. 



(400 c. cm. sea-water -|- 20 drops N/io acetic acid ; 

 number of plutei studied, 50 ; temperature of 

 water, 29° C.) 



Hipponoe (f X Toxopneustes 9 



Toxopneustes cf X Hipponoe 9 



{400 c. cm. sea-water -j- 10 drops N/io liydro- 

 chloric acid.) 

 Hipponoe </ X Toxopneustes 9 



