gyg ALIMENTAEY CANAL. 



to imagine why the more highly organized Spatangoids. which stand at the 

 top of the scale of Echinoids should be the most helpless and least capable of 

 protecting themselves and of choosing their food ; while the Desmosticha can 

 readily change their locality, and are by nature infinitely better protected 

 against the attacks of their enemies than the comparatively helpless 

 Spatangoids. whose main reliance against destruction consists in burrowing 

 in sand-banks, and, by becoming more or less covered by sand, thus escape 

 the notice of their enemies. The Spatangoids have reached such a state of 

 Echinoid perfection that their ultimate disappearance is clearly foreshad- 

 owed ! 



In Rhynchopygus we find the simplest form of alimentary canal observed 

 in Petalosticha. Owing to the delicate nature of the walls of the aliment- 

 ary canal of Spatangoids, it is almost impossible to follow its course in 

 alcoholic specimens. The walls are invariably broken through by the 

 weight of sand which tills the canal. Careful preparations must be made 

 from living specimens to be able to follow its course. A few such prep- 

 arations 1 was able to make at Aeapulco on Rhynchopygus; they will 

 enable us better to understand the more complicated structure of Spatangus 

 as given by Hoffman in his excellent monograph on the anatomy of Spatan- 

 gus purpureus. 1 have freely copied from his memoir the data to elucidate 

 that part of the anatomy of Spatangoids, as we have no common Spatan- 

 goid very available on our coast for study in a fresh condition. The com- 

 parative study of Rhynchopygus, Spatangus. and the Clypeastroids will show 

 a far closer general identity in the structure of the alimentary canal than 

 could have been traced from a comparison of Spatangus with the Clypeas- 

 troids directly. 



In Rhynchopygus the (esophagus is quite narrow, trends slightly to the 

 left of the odd anterior ambulacrum (Pi XXX I If. ;)• takes a sharp turn 

 upwards and opens into a wide alimentary canal (PL XXXII. f. .;). which 

 bends back towards the posterior extremity, running from left to right com- 

 pletely round the edge of the test till it reaches the anterior extremity again. 

 It then makes a second sharp turn upwards and backwards, running again 

 towards the posterior extremity, completely round the edge of the test till it 

 nearly reaches the anterior extremity ; it then curves towards the central 

 longitudinal axis, and, turning completely round, runs in a straight line to the 

 anal opening, which it reaches as a narrow tube compared to the broad 

 pouches which made the last circuit of the test (Pi. XXXII /. 1). At the 



