MONOGRAPH OF THE EXISTING CKINOIDS. 411 



{b) Augmentative regeneration, resulting in an increase in the number of arms. 



1. Duplicative regeneration, resulting in the formation of an axillary from 



which two arms arise in tiie place of a single undivided series of 

 brachials. 



2. Multiplicative regeneration, resulting in the formation of two or more 



axillaries each followed by two series of ossicles in the place of a 

 single undivided series of brachials. 



(c) Reductive regeneration, resulting in the replacement of a lost axillary by 

 a single arm. 



Minckert noted that among the comatulids studied by him from 75 per cent 

 to 90 per cent of the fractures in the arms were at the syzygies. 



In specimens of Isocr/nus Eeichensperger found regenerating buds in the 

 following situations: Seventeen on the distal ends of hypozygals, 10 on the distal 

 ends of IBrj, 7 on the distal ends of radials, and 2 on the distal ends of outer 

 brachials. 



Thus in Isocrimis as brought up in the dredge 75 per cent of the brachial 

 fractures are at nonmuscular articulations (47 per cent at the .syzygies and 28 

 per cent at the synarthries). After the syzygies and synarthries fracture is most 

 common on the distal ends of the radials. 



If an arm be broken off at the radials, necessarilj' a portion of the disk comes 

 away with it. This regenerates first, and for a considerable time shows no 

 trace of ambulacral grooves, which first appear after the new arm has reached 

 an appreciable size. 



Perrier observed that Antedon mediterranea does not thrive in direct sunlight. 

 When the light becomes strong the animals roll up their arms, and if the exposure 

 is too prolonged, or is repeated too often, the ai-ms are cast off. Minckert, bear- 

 ing in mind the fact that the majority of the multibrachiate species live within 

 the zone above the 400-meter line, suggests that possibly it is the influence of 

 the sunlight which induces the young to cast their arms in adolescent autotomy. 

 It is more probable, however, that the arms break off as a natui'al result of 

 growth changes. 



Total loss of the pinnules is relatively rare, but these organs are commonly 

 broken off at the base, or lose their distal portion. This is particularly true 

 of the long oral pinnules when they are stiffened, as in the Stephanometridse. It 

 sometimes happens that a lost pinnule will regenerate a more or less rudimentary 

 arm. 



In types with very prominent disks, which habitually live with the arms widely 

 extended, such as the species of Calometridna, ZygometridiB, and Antedoninse, 

 individuals are frequently dredged in which the original disks have been lost 

 and a new disk is in process of formation. In all the species of Calometridaj, 

 and in the species of the genera Eudiocrhnis and Cafoptometra in the Zygometrida;, 

 eviscerated specimens are usually in the majority, while numerous cast-off disks 

 are found in the bottom of the dredge. Though the first species of Eudiocrinus 

 was described in 1868 , it was not until the last 10 years that the character 



