678 ALIMENTARY CANAL. 



one is the actinal plate ( PI XXXII. f. .;). while in Spatanguswe have an ac- 

 tinal and an abactinal mesenteric plate, both of which consist of a larger and a 

 smaller plate. The smaller actinal plate commences to the left of the solid 

 actinal mesenteric supporter, and reaches the oesophagus on the right. The 

 other larger actinal mesenteric plate covers the whole solid support from 

 which it originates, covering the smaller plate, and follows the winding 

 of the alimentary canal from the junction of the (esophagus to the point 

 where the returning curve of the alimentary canal commences. The dorsal 

 mesenteric plates commence, the smaller one on the posterior left side of 

 the actinal support, following the terminal upper end of the alimentary 

 canal to the anus; the larger plate on the right of the actinal support, also 

 following the edge of the alimentary canal to the aims, and extending under 

 the diverticulum to the returning curve of the alimentary canal, where the 

 actinal and abactinal mesenteric plates meet. 



Hoffman has discovered in Spalangus a remarkable organ, which he calls 

 the "gewundene organ." the function of which is entirely unknown (PI. 

 AAA"//./. 18). It consists of a long tube, somewhat winding, partly follow- 

 ing the course of the large actinal mesenteric plate : it is open at the two 

 ends leading into the alimentary canal, thus forming, as it were, a second 

 smaller alimentary tube. This organ had escaped the notice of all former 

 observers excepting, perhaps. Milne-Edwards, who describes a sort of heart 

 connected with the actinal vessel following the course of the actinal mesen- 

 teric plate. In Spatangoids, as in Desmosticha, we distinguish an inner and 

 an outer layer, and a transverse and a longitudinal muscular layer. The 

 diverticulum probably acts in Spatangoids as an organ of secretion. 



The spicules of the alimentary canal and of the genital organs were hist 

 noticed by Valentin. They consist usually of simple bihamate spicules and 

 of perforated and reticulated plates of considerable size, with irregularly 

 indented margins, extremely variable in shape (PI XX X l'/f /.). To Stewart 

 we owe a more extended notice of the spicules of the alimentary canal 

 and ovaries I PI XXXVIII. f. i .>"'''). which he considers will afford valuable 

 hints for generic and even specific distinctions. As far as my observation 

 goes, I have found these spicules of little value beyond identifying the more 

 extensive groups. The envelope of the genital organs frequently contains 

 large perforated plates, forming sometimes, as in the case of the Cidaris, 

 a regular mailed casing to the ovarian tubes. In the walls of the alimentar}' 

 canal, although plates are sometimes found (PI. XXXVIII. f. .'""'). the 



