MUSEUM OF COMPARATIVE ZOOLOGY. loo 



anterior lobe, passing to the anterior arms. The four arras which arise 

 from the anterior lobe are called the anterior, ar, and the antero-inter- 

 nal, air. Both are furnished with a solid non-latticed central axis or 

 calcareous rod. The anterior rods are listened to the general cal- 

 careous framework of the body ; the antero-internal are free, united to 

 the former by muscular attachments. The larva may be compared to 

 the parts of a chair, the antero-internal rods forming the front legs, the 

 anterior the hind legs, and the oral lobe the back by which these rods 

 are joined to the body of the pluteus. The distal ends of the anterior 

 rods are more widely separated than the antero-internal. The anterior 

 rods are skirted by a ciliated band continued from the antero-lateral as 

 mentioned above. They bear a red pigment spot at a short distance 

 from their distal end. 



The last pair of arms to be mentioned, the antero-internal arms, air, 

 are joined together at one end by the ciliated band which passes upon 

 their rim from the anterior arms, and is joined between them, just as 

 the other end of the circuit is joined on the medial line between the 

 posterior rods. One edge of the junction of the two antero-internal 

 arms forms one of the so-called lips of the mouth, PI. VII. fig. 2. The 

 oral opening, or, is placed between this structure, and the ciliated band 

 joining the two posterior rods or arms. The two antero-internal arms 

 commonly want the prominent pigmentation of the distal ends of the 

 other arms. In one or two specimens, however, pigmentation was ob- 

 served on the distal end of the antero-internal arms. As the calcareous 

 rods which support the antero-internal rods have a separate origin from 

 the rods of the body system, they are capable of movement, and are not 

 fused with the other rods. A number of muscular threads by which 

 this movement is brought about can be seen in the inner angles formed 

 by the rod and the neighboring anterior rods, PI. VII. fig. 18. In the 

 interior of the body we find that the larger part of the body of the plu- 

 teus is occupied by the stomach. Between the stomach and the epi- 

 blastic layer which covers the body of the pluteus many cells are found, 

 some of which are yellow in color. Filaments or threads connect the 

 hypoblastic wall of the stomach and superficial epiblast.* 



* While studying the Embryology of Agalma similar threads were noticed 

 passing from the epiblast of the primitive hydrophyllium to the hypoblastic lining 

 of the same. Similar threads are known in many worm larvas. If these struc- 

 tures are the same as the so-called suspensoria (Selenka) of tlie gastrula, we may 

 reasonably doubt whether Selenka is right in supposing them derived from the 

 mesoderm cells. As far as observation has gone there is no reason to doubt that 



