MUSEUM OF COMPARATIVE 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 cesophagus 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, ov, 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, Pl. 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 well formed, and the oral lobe is 
not yet differentiated into the two oral arms. In a larva four days old, 
Pl. 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 observed 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 cesophagus, 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. 
