A. H. CLARK : THE CRINOIDS OF THE INDIAN OCEAN. 7 



number of new species have been described, chiefly from the collections of the 

 Dutch ship " Siboga." As these new forms do not alter the general conclusions 

 expressed, or the general proportions as brought out by the table, it has seemed 

 best to leave both as originally written rather than to run the risk of error involved 

 in making changes.] 



3. THE DISTRIBUTION OF CRINOIDS IN THE EAST 

 INDIAN REGION. 



In the case of many groups of marine organisms the Indian Ocean and the 

 tropical Pacific, from the east coast of Africa almost to the west coast of America, 

 exhibit everywhere practically the same faunal conditions. The same genera, 

 or even the same species, exist everywhere throughout this great area, and, 

 under similar conditions, are found in the same relative proportions and numbers. 

 A new form first detected in the Hawaiian Islands may next be reported from the 

 Red Sea or from Madagascar, or a new species described from a single specimen 

 taken at Mauritius may prove to be abundant at Formosa or Fiji. But among 

 the crinoids very different conditions obtain. Their sessile habit of life and 

 their fixation as embryos to the pinnules of the adults, and later, as larvte, to 

 the sea floor or to growths upon it, render them incapable of rapid dissemination 

 and have resulted in the demarkation of numerous zoogeographic areas within 

 an area where, so far as we can see, the average conditions are practically every- 

 where the same. 



Before taking up in detail the distribution of these animals it would be well 

 to consider what barriers would be operative against their dispersal, the better to 

 understand the significance of many of the facts brought out. First of all, the 

 very short free-swimming stage of the young, coupled with the limited bathyme- 

 tric altitude inhabited by the adults, renders them incapable of crossing wide 

 stretches of deep water, for before they could drift across they would develop and 

 drop to the bottom, dying as soon as they had reached a depth greater than the 

 lowest limit of their restricted normal habitat; moreover, they cannot cross the 

 mouths of wide and deep rivers; they are very sensitive to a change in salinity 

 and, unless a river be shallow, they cannot pass under it. 



There is a curious connection between the development of a rich httoral 

 comatulid fauna and a copious rainfall which I have explained by assuming that 

 the rain, which has a powerful toxic effect upon most pelagic animals due to the 

 large amount of dissolved oxygen contained in it, kills and precipitates to the 

 bottom a greatly increased supply of the small organisms which serve as crinoid 

 food. This explains the absence or rarity of littoral crinoids on dry coasts. 



The question of food plays a very great part in the local distribution of the 

 crinoids. The small organisms upon which the crinoids feed are mostly lucifu- 

 gous, but are strongly attracted by the rays at the violet end of the spectrum. 

 I have suggested that this accounts for the common purple or violet coloration of 



