MUSEUM OF COMPAKATRE ZOOLOGY. 131 



division extends to the apex of the anal lobe on the posterior side. 

 At the apex of this lobe it subdivides and interdigitates with other cal- 

 careous arms. At about this time or a little later a strong muscular 

 band, well seen in adult plutei, connects the anal rods near the anal 

 pole on the dorsal side. A simple not latticed calcareous rod, ar, bends 

 downward on each side from the common origin of the posterior rods and 

 the most posterior anal body rod, and is continued into the oral lobe. 

 Seen from one side the course of this rod is at first horizontal, until it 

 is about opposite the junction of the oesophagus and stomach, where it 

 sends into the anal lobe a stout anterior anal branch, which extends into 

 the apex of the anal lobe. It there bifurcates, and the divisions inter- 

 digitate with the corresponding divisions of the other rods. The cal- 

 careous rod of the oral lobe, or, is smooth and solid. A straight rod 

 arises from the union of the posterior anal rod with the posterior rod, 

 and extends to the medial line of the dorsal side, ending near the anal 

 opening. Orange and yellow pigment is found in patches at the extrem- 

 ity of the posterior rods, in the anal lobe, and in the anterior lobe. The 

 larva was at times observed to rest on the glass standing on the two 

 posterior arms and the anterior lobe.* At about this time a strong 

 muscular band first appears, which later is very easily seen connecting 

 the anal calcareous rods near the apex of the anal lobe. The object ot 

 this muscle is probably to move the two posterior arms, although the 

 rigid union of these two structures by interdigitation would seem to 

 make any considerable motion impossible. As the larva matures, the 

 body becomes more and more helmet-shaped, approaching the form of 

 the Echinocyamus larva figured by Miiller. 



We are struck, in considering the external form of a pluteus, PI. V. 

 fig. 11, following the last in age, with the size of a protuberance of the 

 dorsal surface, and the inflation of that region upon which the anus is 

 situated. The posterior arms, pr, are weU formed, and the oral lobe is 

 not yet differentiated into the two oral arms. In a larva four days old, 

 PI. V. fig. 7, we see that the anterior or oral lobe has become notched 

 at each corner of the free side of the lobe, and two oral or anterior arms 

 have begun to form. In the stage just before this it was obsen-ed that 

 the posterior arms are longer than the oral lobe. Now, however, the 

 oral lobe has increased in length by the growth of the anterior lobe, the 

 length of which has about equalled that of the posterior rods. With 

 the growth of the anterior lobe the form of the oesophagus, oe, has kept 

 pace. The last mentioned organ is now flask-shaped. 



* This attitude was probably taken on account of the shallowness of the water. 



