234 



PROCEEDINGS OF THE AMERICAN ACADEMY 



Fier. 6. 



The most striking feature in the structure of these small Echini is 

 the position of the anal opening (Fig. 5, a). This is nearly in the 

 central part of the abactinal surface towards the posterior edge, and 

 entirely surrounded by the fasciole. This fascicle, from its position, 

 must undoubtedly be the peripetalous fasciole, as it agrees in position 

 with the same fasciole in Brissopsis, though in the latter genus it does 

 not enclose the anal opening. In the adult Hemiaster the anal open- 

 ing is not thus surrounded, an additional example of the little value 

 we can place upon the position of the anal opening as a systematic 

 character. The transfer of the anal opening to the exterior of the 

 fasciole I was not able to trace, all the specimens being too young to 

 show when it took place. There is no trace in these young stages of 

 any genital openings, or of genital plates; the ocular plates are 



somewhat more prominent than the 

 other ambulacral plates, one speci- 

 ally, that of the odd ambulacrum 

 (see Fig. 5). On opening one of 

 these young Echini (Fig. G), we 

 find that, notwithstanding the posi- 

 tion of the anal opening, the intes- 

 tine already makes a half circuit 

 round the edge of the test, and is 

 attached to the sides by the usual 

 mesenteries, the actinal extremity of 

 the alimentary canal towards the an- 

 terior end being free ; the stone canal also leads nearly vertically 

 flora the anal o^iening to a terminal interambulacral plate situated 

 to the right of the odd ambulacrum. The anal opening is large, 

 pentagonal, separating completely the trivium from the bivium, and 

 is covered by a large plate having a small opening opposite the left 

 posterior ambulacrum. 



The only other young Spatangoid known, resembling so closely a 

 regular Echinus, is a young Spatangoid figured by Miiller, while still 

 in the Pluteus stage, with straight spines similar to these figured here 

 in the youngest specimen. This was the first indication we had of the 

 great similarity of the sjjines of the young stage.^ in the regular and 

 irregular Echini. The presence of an anal opening in the young 



Fio. 6. Section of Hemiaster, showing tlie course-of the alimentary canal c', 

 from the mouth m, to tlic anal system an ; c, the stone canal, extoiuling 

 from tiie circuhir ring to madreporic body md. 



