136 BULLETIN OF THE 



The apex of the anal lobe is densely pigmented, and the walls are 

 supported by highly ramified divisions of the anal rods (posterior and 

 anterior), which interdigitate and form an intricate network. An anus 

 is present, and the intestine is elongate or flask-shaped. The mouth is 

 commonly widely open, circular, with ciliated lips. The CESophagus is 

 densely ciliated on the interior walls. The pluteus moves from place to 

 place easily but not rapidly, and is just visible to the naked eye. Tlie 

 length is .85 mm. ; diameter of the body .20 mm. 



One of the most striking differences between the adult pluteus of Echi- 

 narachnius and A. Agassiz's drawing of that of Strongylocentrotus of the 

 same age, is the existence of large pigment spots near the distal end of 

 each arm, while the anterior and antero-internal arms of Strongylocen- 

 trotus have no 3uch spots, at least of the great prominence which we 

 find in Echinarachnius. The pluteus of Strongylocentrotus, according 

 to A. Agassiz, has ciliated epaulettes. These structures are not repre- 

 sented in the pluteus of Echinarachnius. Like the pluteus of Echino- 

 cyamus, as figured by M tiller, Echinarachnius has no ciliated epaulettes. 

 The reseml)lauce of the pluteus of Echinarachnius to that ascribed to 

 Echinocyarnus is very great. If we compare the figures given by Miiller 

 and those of the Echinarachnius pluteus here figured, we notice one or 

 two marked differences between them. The arms of the pluteus are 

 much longer and larger in Echinarachnius than in Echinocyamus. The 

 posterior and antero-lateral rods of both genera are latticed. The an- 

 tero-lateral and antero-internal in both are not connected with the body 

 system of rods. The anterior lobe before the origin of the antero- 

 internal arms is longer in Echinocyamus than in Echinarachnius. The 

 characteristic pigment spots of the ends of the arms of Echinarachnius 

 are not found in Echinocyamus. The difference of the young from the 

 adult sea-urchin formed from the pluteus of Echinocyamus has at- 

 tracted the attention of Miiller. The young Echinarachnius raised 

 from the pluteus is somewhat different from Miiller's figures of the 

 young Echinocyamus.* 



the snspensoria are mesodermic, as Selenka says. In Agalma these structures 

 appear to be epiblastic. It may be said, however, that they originate from the 

 epiblast, just as the mesoblastic cells themselves may originate as simple exten- 

 sions and buds. The homology, therefore, of the suspensoria and the filament in 

 the primitive Iiydropliyllium cannot be made out at present. It may be said that 

 the likeness between the two is great. (For filaments in the primitive larva of 

 Agalma, see Embryology of Agalma, Bull. Afus. Comp. Zool., XI. No. 11.) 



* It is taken for granted that tlie pluteus described by Miiller is an Echinocy- 

 amus, although he did not raise it in the egg. 



