746 ON THE YOUNG STAGES OF ECHINI. 



f. /) of Strongylocentrotus is in marked opposition to the old specimens 

 [PI. X.). The different stages of growth of Arbacia (PL V.) differ more 

 than many of the genera recognized among Echini. The successive stages of 

 growth of Toxopneustes are very unlike. The young Temnechinus (PL V. 

 III. f. 16), although we have traced its gradual passage to the stage of PI. 



VIII. f. l), will always seem most closely allied to Arbacia. We naturally 

 refer the striking phases of growth to forms with which we are familiar, 

 and which, correctly or not. have been recognized as independent genera. 

 Hence we are led to draw the inference either that the widely different 

 forms are genetically connected because their analogues have apparently 

 passed during their embryonic growth from one stage to the other, or that 

 it is still possible, even in widely differing forms, that a genetic connection 

 should exist, owing to the sudden transitions which we see going on during 

 the development of an individual. 



The Scutellina stage \ PI. XI. f. i) of Mellita seems at firsl glance very 

 distinct from the Monophora (PI. XL f. s), from the Kavenellia (LuTK.) 

 I PI. XL f. in), or from the true Mellita stages, yel on a close analysis these 

 stages are all due to very slight Structural changes, although they are so 

 different at firsl glance and apparently are not linked by intermediate stages. 

 if examined simply as we should examine them on the supposition of their 

 being adults. The petaloid and simple ambulacra only differ in quantity ; 

 the mere spreading of the median interporiferous /.one produces differences 

 in the ambulacra! system which have been used to distinguish not only 

 genera, but even families. This is especially the case in the CTypeastroids 

 and Spatangoids, and shows that the (laleritidae. apparently so distinct 

 from all the other Kchini. are most closely allied to the Clypeastroids. hav- 

 ing their tubercles, their disconnected anal system, the simple ambulacra 

 dl' the young Clypeastroids. and having besides the notches of the actinal 

 system of Echini proper. The position of the anus, a most striking feature, 

 is of little consequence, as during the growth of the young Echinus we find 

 it passing from an actinal to an abactinal position, and it would be most 

 natural to separate widely forms in which the anal opening should in one 

 case be situated next to the mouth, as it is in the stage immediately follow- 

 ing the pluteus stage, and in the other when placed at the opposite pole 

 from the mouth, as in our Strongylocentrotus. The unstable position occu- 

 pied by the anal opening is well shown in the different stages of Echina- 

 rachnius (PI. XII.). 



