THE INTERAMBULACRUM. 63 



of these five parts. Such an ideal area composed of plates 'associated with a single ocular is 

 shown in Palaeechinus and Eucidaris (text-figs. 217, 218, p. 191). Support for this view is found 

 in the fact that the perignathic muscles of the lantern, which are nearly or quite ambulacral 

 in position, are inserted wholly on the base of the associated two half-interambulacra, as in 

 Eucidaris (text-fig. 218); or if not, they are inserted on two half-interambulacra and auricles 

 of the ambulacrum itself, as in Centrechinus (text-fig. 219, p. 191). 



In the Echinothuriidae where oculars and genitals are separated by an interspace, Mr. 

 Agassiz (1883, p. 32; 1904, p. 96), says that the interambulacral plates are derived from the 

 periproct, as the two areas are there in contact. Dr. Mortensen (1903, p. 175) opposes this 

 view. Evidently this contact is a mere coincidence of structure and has no bearing on the 

 origin of plates. In the echinothuriids which I have seen, the young interambulacral plates 

 are in contact with the oculars as usual (text-fig. 170, p. 149). Wliile in regular Echini outside 

 the Echinothuriidae the oculars and genitals typically form a closed ring, yet occasionally in 

 variants the ring may be open so that the periproct reaches the interambulacrum, as in 

 Strongylocentrotus (Plate 5, fig. 15; Plate 6, fig. 5). 



The full differential characters of the interambulacrum as of the ambulacrum are expressed 

 at the mid-zone of the adult. Here are usually found the full number of columns of plates 

 characteristic of the species, also the typical tubercles, spines, imbrication, or other characters 

 which go to make up the specific description. The ventral border in the basicoronal zone 

 represents the earliest formed plates and the youth of the individual, as far as it can be gathered 

 from the study of an adult specimen, though the actually first formed plates may have been 

 resorbed in development. Passing dorsally, with later added plates, new characters may 

 come in until we get the full differential features developed at or about the mid-zone. Dorsal 

 to the mid-zone we pass into the area of young last formed plates which have not yet acquired 

 the full characters. Or again dorsally, we may find senescent features in the loss of columns 

 of plates. Passing from the basicoronal row dorsally, we find in most Palaeozoic types, and 

 many post-Palaeozoic as well, stages in development strongly marked, which stages can be 

 correlated with the adult condition of simpler genera or simpler species within the genus. The 

 interambulacrum in Echini has from one to fourteen vertical columns of plates in each of the 

 five areas, which represents the least and greatest number known at present. There are 

 intermediate grades representing every step between this least and greatest specialization 

 of the area, and it is a matter of great interest to follow the progressive series as represented 

 by stages in development, and by adult types, to see how the progressive differential structure 

 is built up. As the plates of the ventral border are the oldest or first formed of any plates 

 seen in an individual specimen, and as the later added plates succeed one another as we pass 

 dorsally, it might be thought that we could read stages in development as expressed by rows 

 and columns of plates with ease and certainty, and such can be done in many types, as in 



