20 BULLETIN OF THE 



condition they are protected by the cap formed by the sides and dorsal 

 region of the terminals, and, as the terminals are pushed out by growth 

 of dorsals and marginals, new interambulacrals continually form under 

 the shelter of the terminal. The law of development of ambulacrals 

 and marginals, in the arm holds in the interambulacrals. The oldest 

 formed are those nearest the mouth ; the youngest are the nearest to 

 the terminals. The beginnings of at least three pairs of ambulacrals 

 are formed before the first true interambulacral appears. The marginals 

 antedate the interambulacrals. 



There is little variety in the progress of the growth of the interam- 

 bulacrals, from the time they first appear as small calcareous nodules 

 (PI. III. fig. 2) until they form the compact blocks of older stages in the 

 growth. They differ from most of the other plates of the starfish in their 

 massive growth, and they never have the flat perforated plate form of the 

 dorsals or genitals. As they mature, they fit closely together, forming 

 square blocks with re-entering angles, and are closely articulated. They 

 also carry a single long spine in early stages. 



In younger forms of the starfish, before the ambulacrals have be- 

 come so crowded that there are four rows of feet, the interambulacrals 

 alternate with the ambulacrals. At that time the number of interam- 

 bulacrals is the same as that of the ambulacrals, with the exception, 

 however, of the newly forming ambulacrals at the extremity of the 

 ray. In all young starfishes the single row of interambulacrals stands 

 out clear and distinct from the other plates, while their number always 

 has a constant relation to that of the ambulacral rafters. 



Marginal Plates. — Large and important plates of the arms, origi- 

 nating early in the development of the larva, may be known as the 

 marginal plates, m. These plates are formed at the extreme end of 

 the ambulacral rafters, between the adoral rim of the lateral extensions 

 of the terminal plates and the interbrachial region of the body. They 

 follow the same law in sequence of formation as the adambulacral, but 

 do not have the protection of the terminals in their early condition. 



The first marginals to form appear in a stage between one with a 

 single median dorsal and one with two median dorsals (PI. IV. figs. 1, 2). 

 It is a curved plate, extending on the actinal side to the interbrachial 

 region and on the abactinal to the vicinity of the first median dorsal, d. 

 On its dorsal region it bears a single spine (PI. IV. fig. 2). 



The first pair of marginals is firmly jammed in between the lateral 

 extensions of the terminals and the interradial portion of the circumoral 

 plates, and by its subsequent growth helps to push out the terminals in 



