MUSEUM OF COMPARATIVE ZOOLOGY. 141 



logons to the other plates of the arms, and if so, to which, — dorsals or 

 laterals or ventrals ? In its mode of origin it is very much like a dorsal. 

 It arises in the median dorsal line ; it is single. It grows on the sides 

 of the vessel as a lateral. With the ventral it has no resemblance. If 

 it were possible to carry the homology of plates so far that every pair of 

 adambulacral plates should have corresponding dorsals, it would be neces- 

 sary to regard the primary radials as the dorsals of the first pair of radials, 

 and then the terminals might be regarded as the dorsals of the second pair 

 of adambulacrals. The third pair of adambulacrals would then be rep- 

 resented by true lateral plates, with a dorsal in the arm itself. This sup- 

 position and all similar theories seem to be overthrown by the fact that 

 the terminal in the growth of the arm is pushed to its tip, far away from 

 the plates with which it occupies relationships in early life. Whether it 

 will be found that the terminal is a modified dorsal or lateral plate, or 

 a plate unrepresented in other regions of the arm and disk, its fate in 

 the formation of the arm is unique. While its early structure as a 

 spicular calcification on the median radius resembles the radialia and 

 the dorsal plates, its form in the adult is very different from these 

 structures, and its relations to the water-vessel such as no other plate of 

 disk or arms has. 



Side Plates. — The side plates (Ijj) of the arms are regarded as adam- 

 bulacral in their homology. They originate laterally and in pairs, grow- 

 ing dorsally and ventrally until they approximate dorsals and ventrals. 

 The oldest side plates are adoral, and new side plates form between those 

 already present and the terminals. The first pair of side plates originate 

 after i\\Q first ventral {V), Siudi before the arm has begun to push out. 

 They therefore appear when first formed in the wall of the disk. Spines 

 arise from the aboral edge of the side plates, those on the first pair of 

 side plates being well formed before the fourth pair of side plates have 

 appeared (Fig. 20). The time of the appearance of the side plates as 

 compared with the first ventral will be seen, by a comparison of the above 

 statement with that of Ludwig, to be contradictory. Ludwig says : * 

 " Es entstehen aber die zu einem Armgliede gehorigen Seiten- Dorsal- und 

 Ventralplatten nicht etwa auf einmal sondern zuerst sich nur die Seiten- 

 platten an." While both V and V^ (PI. XI. Fig. 18, op. cit.) are regarded 

 by him as ventral, especial attention is turned by Ludwig to V^ as the 

 first ventral, in his reference to the sequence of the formation of the side 

 plates and ventrals. I believe that the contradiction pointed out between 

 Ludwig's account of the sequence of these plates and my own is apparent 



* Op. cit., lip. 188, 189. 



