44 BULLETIN OF THE 



arc not homologous with the oculars of the Urchin. Whether there are 

 plates in the young urchins which are homologous with the terminals 

 of the starfishes is a question upon which more knowledge of the devel- 

 opment of the calcareous plates of the Echinoids may throw some light. 



According to Ludwig (p. 188) * A. Agassiz " halt die jungen Termi- 

 nalia fur die Dorsalschilder des Erstgebildeten Armgliedes, eine Bezei- 

 hung die ebenfalls nicht korrekt genau ist." Agassiz says, "The 

 only calcareous deposits we have {y if, fig. 32) are evidently parts of 

 the first arm joints, the dorsal (ij, fig. 32), and the side arm shields 

 (f, fig. 32) of that joint, which consist at present of but a few rods 

 indicating their future position." (Embryology of Echinoderms, p. 20.) 

 This was written of an Ophiuran with a pluteus, which is probably 

 Ophiopholis. Agassiz uses the term "arm joint" elsewhere in his pa- 

 per to designate one portion or section of the arm. I am unable to 

 understand exactly what Ludwig means by the above criticism of Agas- 

 siz, unless it is that the terminals (dorsal shields, Ag.) do not belong 

 to the first formed arm joint, or possibly that the terminals do not 

 indicate an arm joint. One of these interpretations is the best I can 

 make of his meaning. If we consider an arm joint among the Ophi- 

 urans to mean a portion of the arm with a dorsal, two laterals, and 

 a ventral with enclosed organs, the portion of the ray in which the 

 terminal lies might not be called an arm joint, since separate calci- 

 fications for lateral and ventral plates do not exist. However the 

 terminal may be homologized, it originates like a dorsal, and grows 

 around the terminal tentacle forming laterals and ventral. In the 

 structure of the plates, as in position and time of origin, it differs from 

 all other dorsals, and consequently morphologically may be held by 

 some not to be an arm joint, or to belong to the first formed arm 

 joint. Possibly that may be the meaning of Ludwig's criticism above 

 quoted. If not, I have been unable to see the force of his criticism. 



In speaking of an early stage of Asterina in which the first dorsals, 

 or " radials," are formed, Sladenf says : " The first formed plates in 

 the viviparous or abbreviated larva are the primitive elements of the 

 terminal plates, the basal plates, and the dorsocentral plate. These 

 become well developed before any traces of the radial plates make their 

 appearance. Concurrent with the radial plates the lateral plates (inter- 

 ambulacral plates) are developed." He might have added also certain 

 mouth plates, ambulacrals, and interambulacrals (adambulacrals). 



* Entwicklungsgeschichte der Asterina gibboso, Forbes, 

 t Sladen, op. cit., p. 31, fig. 15. 



