ASTERIAS PALLIDA. -^^^ 



with each other in the prœoral lobe and send out from their anterior 

 •extremity three diverticula into the rudiments of the brachiolar arms, 

 which are just visible as out-pocketings of the body-wall. The pos- 

 terior extremity of tlie left anterior enteroccel is directly con- 

 tinuous with the rudiment of the aquiferous system ; indeed there 

 is no natural boundary between the two at this stage, and we can only 

 draw an arbitrary line on the basis of subsequent results. The pore- 

 canal, which, be it remarked at the outset, I look upon as belonging 

 to the enterocœl and not to the hydrocœl, as will become clear at a 

 later stage of our description, is very short and opens into the postero- 

 dorsal corner of the left anterior enterocœl. Its external opening is 

 on the right side of the median line. The left posterior enterocœl 

 is U-shaped in both its lateral and ventral aspects, being most spacious 

 at the posterior end of the body. The ventral horn of the U as seen 

 from the left side extends just posterior to the anus and here curves 

 backwards on the right side and ends blindly a little before reaching 

 the posterior end of the body, its wall being closely appressed to that 

 of the right posteri(3r enterocoel. The latter looks somewhat spacious 

 in its lateral aspect ])ut is considerably compressed from side to side. 

 It does not, moreover, extend very much ventrad, owing to the 

 encroachment of the left posterior enterocœl on the right side. Ex- 

 cepting in the prasoral lobe, the cavities of the two sides of the body 

 are separated by a mesentery on the wliole of the dorsal, and the greater 

 part of the ventral, median line. At the posterior portion where 

 the left cavity extends to the right side, the mesentery is removed to 

 the right to a corresponding extent. 



The rudiment of the hydrocœl is present in the form of five out- 

 pocketings of the posterior portion of the left anterior enterocœl, and 

 of a central cavity enclosed by connecting the bases of these diverticula. 

 These out-pocketings are, it is needless to say, the rudiments of the 



