REPORT ON THE ECHINOIDEA. 181 



the interambulacral area being clearly marked by the two large tubercles at the extremity 

 of these areas on the abactinal edge of the fasciole. The whole fasciole was covered by 

 a coarse granulation. The most striking feature in the structure of these small Echinids 

 is the position of the anal opening (PI. XX.'' fig. 9). This is nearly in the central part 

 of the abactinal surface towards the posterior edge, and entirely surrounded by the 

 fasciole. This fasciole, 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 opening 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 es23ecially, that of the odd ambulacrum (see PI. XX." fig. 9). On 

 opening one of these young Echinids (PI. XX." fig. 11) we find that, notwithstanding the 

 position of the anal opening, the intestine akeady 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 anterior end being free ; the stone canal also leads 

 nearly vertically from the anal opening 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 liy a large plate having a small 

 opening opposite the left posterior ambulacrum. 



" The only other young Sjjatangoid known, resemljling so closely a regular Echinus, 

 is a young Spatangoid figured by Mliller, 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 simUarity of the spines of the young stages in the regular 

 and irregular Echinids. The presence of an anal opening in the young Hemiaster 

 connected, so to speak, with the abactinal system, is a most interesting feature, as well as 

 the complete separation of the bivium and trivium, the origin of which among Echinids 

 had not been understood. The whole family of CoUyritidse, in which this is the normal 

 state, appear in geological times as an abnormal group, disconnected entirely, and 

 isolated from all the other Spatangoids, which it precedes in time, and seeming thus far 

 to have no connection with the Spatangoids of later geological periods. Theii" connection 

 as an embryonic stage is now clearly shown by the young of Hemiaster, here figured, as 

 well as the close relationship existing between the regular Echinids and such Spatangoids 

 as Collyrites, appearing as the earliest geological representatives of the Spatangoids. 

 The CoUyritidae are, therefore, not structurally so far removed as has been generally 

 supposed from the regular Echinoidea. 



" The earlier development, that preceding the stage when the embryo escapes into 



