SHALLOW-WATER STARFISHES 33 



as in those of Pisaster, the number of interpolated interactinal rows 

 (sa) goes on increasing during the whole or most of the period of 

 growth, so that in large specimens there may be four or five rows on 

 each side. But in many other species the number of rows is fixed 

 very early, the young, two inches in diameter, having as many rows 

 as those six to eight inches in diameter, the subsequent increase in 

 size being due to the enlargement of the plates and the formation 

 of new ones dorsally and distally. But in many species with long, 

 slender rays the peractinal plates alone are developed, and in a few 

 even these are lacking, or else so small as not to be visible exter- 

 nally, as in Urasterias linckii, etc. So, likewise, in other long-rayed 

 species, the dorsal skeleton may lack more than one pair of interpo- 

 lated rows of ossicles, as in Coscinasterias. 



B. INTERACTINAL PLATES AND SPINES. 

 Text-figure i. 



The presence or absence of actinal plates and spines, and whether 

 one row or several rows of these plates be developed, are characters 

 that often seem to be of generic value. Those forms that have no 

 such plates at maturity, or have only one imperfect row, are probably 

 the more primitive types, or at least they have probably inherited and 

 preserved this primitive character. The very young starfish in all 

 the genera apparently has none of these plates, but acquires them 

 as growth proceeds. They are lacking or rudimentary in Urasterias, 

 Distolasterias, Stylasterias, and some other groups. They exist in 

 only one row in Orthasterias, Coscinasterias, etc. In true Asterias 

 and some other groups, one or two, or more, incomplete subactinal 

 rows are usually added. In Evasterias, Pisaster, and Cosmasterias, 

 two, three, or more rows are present. In those species that have 

 subactinal plates, these increase in number during the whole period 

 of growth, and new rows may also be added continually. Never- 

 theless these plates seem to afford morphological characters of much 

 value in classification, and they are usually coincident with other 

 special features. It is of great importance that correct distinctions 

 should be made between the actinal, infer omarginal, and supramar- 

 ginal plates and spines. This has not been done in most of the 

 earlier descriptions. Side views of the rays are necessary to show 

 these plates properly, but such views are rarely given. In many 

 species they are without spines and so concealed by thick skin that 

 they cannot be determined without preparation, and therefore are 

 not visible in alcoholic specimens. 

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