Growth-changes in Brittle Stars. 95 



whose disk exceeded 5 mm., occurring by the hundred. In the same sponge 

 there also lived, although not in anything like the same abundance, the 

 widely distributed and well-known Amphipholis squamata (Delle Chiaje), 

 whose early development and metamorphosis have been studied by both 

 Fewkes (1887) and Ludwig (1881). I secured a good series of this species 

 and, owing to its viviparous habit, was able to get some very young material. 

 The third species of brittle-star of which I secured young also inhabited 

 this red sponge and the adults and half-grown specimens were very abun- 

 dant. It is Ophiothrix angulata (Say), a well-known West Indian species 

 of a cosmopolitan genus, particularly interesting because it is the type genus 

 of one of the very few well-characterized families of brittle-stars. Appar- 

 ently Ophiothrix was not breeding, as Ophiactis and Amphipholis appeared 

 to be, and the young were very scarce. But enough were obtained to throw 

 considerable light on the post-larval development of the species. 



METHODS. 



When the abundance of Ophiactis in the red sponge was discovered, it 

 was hoped that they could be kept in aquaria and studied as living material, 

 but this proved to be impracticable. If they were removed from the 

 sponge and put in clean water by themselves, they soon became sluggish 

 and died in a comparatively short time; few lived more than 24 hours. On 

 the other hand, if pieces of the sponge were placed in aquaria, the water 

 became foul with astonishing rapidity and the death of all animals speedily 

 ensued. The observations made on the brittle-stars in life were therefore 

 of little importance. Material was preserved for further study by the very 

 simple method of killing with alcohol, or with formalin in which corrosive- 

 sublimate was dissolved, after the animals had been narcotized with mag- 

 nesium sulphate (Mayer's method). The best material proved to be that 

 which had been killed and preserved in alcohol. As the calcareous plates 

 were the parts particularly desired for study, of course any acid reagent 

 was out of the question. Work on the preserved material has been greatly 

 facilitated by the use of sodium hypochlorite. This alkaline reagent is so 

 powerful a solvent of organic matter that when used in full strength it will 

 reduce a small brittle-star to a heap of calcareous particles in a few minutes. 

 But as it may be diluted with water and mixes readily with glycerine 

 without affecting the "clearing" properties of the latter, it can be perfectly 

 controlled, and the solution of the organic matter and the separation of the 

 calcareous plates can thus be accomplished as rapidly or as slowly as one 

 wishes. Usually glycerine served as a clearing agent, but occasionally 

 better results were obtained from the use of xylol. The latter, however, 

 will not mix with the hypochlorite. 



Attention has been centered on the skeleton, since it is well-known that 

 in all echinoderms, except holothurians, modifications of the skeleton almost 

 always accompany changes in any of the soft parts. This is particularly 



