PART 5 A MONOGRAPH OF THE EXISTING CRINOIDS 389 



Salinity range. On the coasts of eastern Greenland there are six records of salinity 

 ranging from 32.825 to 34.93 parts per thousand. On the Murman coast and Barents 

 Sea area, Scheuring gives six records varying from 33.33 to 34.88 parts per thousand 

 and Stuxberg has one from the Gulf of Obi of 32.2 parts per thousand. 



There are 16 density records from the north of Russia, ranging from 1.0238 to 

 1.0275, the average being 1.0252. 



Bottom. This species occurs almost exclusively on soft or loose bottom, mud, sand, 

 gravel, or loose stones, sometimes upon any one of these alone, but usually on two or 

 more mixed in varying proportions. Perhaps the most typical bottom is gravel with 

 sand and mud. 



Occurrence. Heliometra glacialis is apparently more generally distributed and 

 more abundant about Spitsbergen and eastward than it is in the Greenland area and 

 westward, probably owing to the existence there of much wider areas of sea bottom of the 

 character especially suited to it found within the optimum ecological limits. 



It grows to a large size only in the colder portions of its range, especially on the 

 coasts of Greenland and hi the open sea about Spitzbergen, and in the northern part of 

 the Barents Sea and eastward. From Labrador to Massachusetts the size is always 

 rather small, as hi the region of the Faroes; about Iceland the size is even less, while 

 on the Murman coast and in the southern Barents Sea it is represented only by what 

 appears to be a curious dwarf variety. 



Throughout its range it is, for a crinoid, most exceptional in the relative evenness 

 with which it is distributed over the sea floor, resulting probably from its peculiar 

 adaptation to life on the very extensive and characteristic arctic bottoms of hard mud, 

 gravel and stones of various sizes hi its very numerous cirri, which prevent it from 

 sinking into mud, and hi the great size of these organs, which enables it to cling securely 

 to a very rough bottom. 



The difference in frequency of occurrence between this species and the second 

 characteristic Arctic comatulid, Poliometra prolixa (see p. 588) is most striking, and is 

 probably due to the inability of the latter to exist on stones or rough ground because of 

 the delicacy and fragility of its cirri. 



While occasionally mixed lots of this species are brought up by the dredge including 

 large adults, young, and even pentacrinoids, as a general rule there is a marked tendency 

 for the animals to be associated hi definite age groups so that at any one station only 

 individuals of approximately the same size are obtained. 



As under different conditions, particularly of food and temperature, these, like 

 most animals, to a greater or lesser extent show hi the young a different correlation in 

 the relative development of the different organs and structures though in the fully 

 grown essentially the same condition of stability appears everywhere to be attained, 

 there is commonly an appreciable difference between specimens of the same size from 

 different localities. 



For instance hi two lots of similar half -grown specimens those hi one lot may show 

 already developed the form of the brachials, the interrelationships of the lower pinnules 

 and the shape and proportions of their segments characteristic of much larger individ- 

 uals, while those in the other lot may present these features in a form characteristic of 

 the ordinarily much smaller young. Or these features may be combined in various 

 ways, the brachials being of the kind found in the fully grown but associated with lower 

 pinnules of the kind characteristic of the young, etc., and they may be thus mixed not 



