MUSEUM OF COMPARATIVE ZOOLOGY. 11 



In all the stages which have been studied no embryonic plate or 

 plates were found which are at one time well formed and later lost by 

 absorption. I have looked for these plates especially along the me- 

 dian actinal line of the arm, but was not able to discover tliem. The 

 large size of the spines would seem to stamp them as embryonic organs. 

 Certainly the size of the spine on the dorsocentral or on the terminals 

 is relatively much larger than in the adult and older stages. This pre- 

 dominance in magnitude of these appendages does not necessarily mean 

 that they are later lost, but it simply indicates a relationship of Asterias 

 in its youth to spiniferous forms. The later grosvth, when new spines 

 form, simply brings the spine to lose its relatively great size on account 

 of a corresponding growth of the plate and other spines. 



3. Development of Plates, Rods, Pedicellariae, Spines, and 



Stone Canal. 



The following calcifications are considered in this place : — 



Body. 



Ahactinal hemisome. 



1. Dorsocentral (Centrale, Luclwig), dc. 



2. Genitals (Basals, Sladen, Interradialla, Ludwig), rp- -g^. 



3. Plates on the radial line, dd^. 



4. Abaxial interradials, gg, gg^. 

 6. Connectives,* c. 



Actinal henmome. 



1. Ambulacral orals, t am. 



2. Interambulacrals J (Adambulacrals, aurt.), ad-ad^. 



3. (Odontophores §) ; First Interbracliials, auct. 



4. Interbracliials, auct. 



Arms. 

 Ahactinal. 



1. Terminals, t'^-i^. 



2. Dorsals il (Intermediare, Lndwiq), d-d'^. 



3. Marginals (Interambulacrals, Ludwig), m-m7. 



4. Dorsolateral, dt. 



5. Connectives, c. 



* By the connectives the smaller plates connecting the larger are meant. 



t I should prefer the term Oral, if the name had not been applied to certain 

 other plates. 



J It may be as well to retain the old term, especially as they arise between 

 ends of successive ambulacrals. 



§ An unfortunate designation. I prefer Interbracliinl. First bomologized by 

 Ludwig with oral, later {op. cit.) with unpaired marginal (Unparige Interanit)ulacral). 



II The first of the dorsals is bomologized with radial of crinoid by Sladen. 



