ACTINOSTOME. 



There is a marked difference in the position of the actual opening of the 

 mouth in those Echini which have teeth, as in the Desmostieha and Clypeas- 

 troids, and in those which are edentate, as in the Petalosticha. The actual 

 mouth is a circular opening of the actinal membrane, from which the five 

 jaws protrude (PI. VII. f. w); the actinal membrane fitting closely to their 

 outer sides, while on the inner side, within the jaws, where the teeth arc 

 attached to the pyramid, is found the inner opening, formed by the attach- 

 ment of the continuation of the walls of the alimentary canal within the 

 pyramid. This continuation forms a sort of (esophagus, which in the toothed 

 Echini is placed either wholly or in part within the pyramid of the jaws, and 

 separates the peristomal cavity from the exterior. This short oesophagus 

 is only attached at the extremity, and is separated from the upper part of 

 the pyramids, leaving them full play for motion ( PL XXVII. f. 1. .:). 

 Valentin lias given an excellent figure of the intestine separated from the 

 pyramid, on PI. VII f. 122 of the Anatomic du Genre Echinus. In the 

 Clvpeastroids. owing to the shorter height of the jaws, this oesophagus is quite 

 rudimentary, hut the mode of attachment of the inner mouth membrane and 

 of the actinal membrane to the pyramids is the same a- in the Desmostieha 

 (PI XXIX. bwerfig.; PI. XXXVIII f. i. : ; PI. XXX. upper fig.). 



In Spatangoids the opening of the actinal membrane leads directly into the 

 oesophagus through a more or less conical pouch (PI. XXXII. f. 5,11,17) 

 in which we do not find the muscles and folds so characteristic of this part of , 

 the digestive canal in the toothed Echini. The actinostome of some of the 

 Spatangoids consists of a transversely elliptical opening, in which an upper 

 and a lower lip can he (PI. XIX". /'. .;, 6, S, 9) distinguished ; the actinal 

 membrane is covered by large polygonal plates, leaving a small mouth-open- 

 ing ; the plates are largest near the anterior extremity, becoming smaller 

 and frequently very minute near the actinal opening (PI. XXI". f. 3 ; PL 

 XXXII. f. ii ; PI. XXXIV. f. .'. j). By the movement of the upper lip 

 the actinal opening can be increased or decreased, and even closed. The 

 upper and lower lips of the actinostome are well marked in such genera 



