A MONOGRAPH OF THE EXISTING CRINOIDS 65 



in a specimen of Compsometra parviflora with 10 arms 25 mm. long, in which the 

 grooves measure 1.37 meters. It is only inconsiderably longer in a specimen of 

 Pentametrocrinus diomedeae with 5 arms 55 mm. long (1.85 meters) and in a specimen 

 of Thaumatornetra comaster with 10 arms 45 mm. long (2.98 meters). Oligometra 

 chinensis with 10 arms 55 mm. long has the ambulacral furrows 7.15 metors in length, 

 while a large Antedon petasus with the 10 arms 110 ram. long has them 16.28 meters 

 long. A specimen of Stephanometra spicata with 26 arms 130 mm. long has the ambu- 

 lacral grooves 38.48 meters long; a specimen of Liparometra grandis with 26 arms 120 

 mm. long has them 44.72 meters long; a specimen of Heliometra glacialis with the 10 

 arms 200 mm. long has them 54.5 meters long; and hi a specimen of the multibrachiate 

 Himerometra magnipinna with 49 arms 90 mm. long they reach 61.23 meters in length. 

 A large individual of Metacrinus rotundus with 56 arms 210 mm. in length from the 

 radials has the ambulacral furrows 71.68 meters long. The highest numbers, how- 

 ever, are found in the Comasteridae, a specimen of Comantheria grandicalyx with 68 

 arms 125 mm. long having the ambulacral furrows 102.68 meters in length. 



Gisl6n mentioned a few facts that it seemed to him might influence the relative 

 length of the ambulacral furrow. At great depths the food supply is more scanty 

 than higher up, partly because the bottom life is not so abundant, and partly because 

 the plankton does not rain down so plentifully, whereas in the littoral zone the food 

 is very abundant. Deep-sea forms ought therefore to have a relatively long ambu- 

 lacral groove in proportion to their size, while shallow water forms ought to be able 

 to manage with a shorter one. The degree of calcification probably plays some part 

 also. A strongly calcified animal has relatively less organic substance tban one which 

 is inconsiderably calcified. The former has less substance to support, and ought to 

 be able to obtain the necessary amount of nourishment with a proportionately 

 shorter ambulacral furrow. 



Two objects of similar shape but differing in size stand in relation to each other 

 as the square of the surface and the cube of the volume. If we take a 10-armed 

 comatulid of a certain size and compare it with a similar but much smaller one the 

 lesser one ought to have a relatively greater surface in proportion to the volume ; or if 

 we compare it with a larger one, the larger one in proportion to the volume ought to 

 have a smaller surface. The figure represented by the total length of the ambulacral 

 groove is a function of the surface supplying nourishment and varies with it. The 

 ratio between the ambulacral grooves and the volume in very small comatulids gives 

 an abnormally high figure, and in very large comatulids an abnormally low one. 



The following figures show the relationship between the length of the ambulacral 

 furrows and the bulk in various comatulids. 



Comissia panmla, Sagami Bay, Japan, 720 meters (10 arms 35 mm. long) : Length 

 of ambulacral furrows, 1.95 meters; weight 0.89 grams; volume 0.74 cc; specific 

 weight 1.20; length of ambulacral furrows divided by volume 2.64. 



Comatula pectinata, Cape Jaubert, Western Australia, 20 meters (10 arms, 6 

 grooved, 90 mm. long) : Length of ambulacral furrows 4.37 meters; weight 4.83 grams; 

 volume 3.46 cc; specific weight 1.39; length of ambulacral furrows divided by volume 

 1.26. 



