370 SOME ZOOLOGICAL METHODS. 



mass, or some other mass that does not necessitate treatment 

 with alcohol (which dissolves out the pigment, leaving the 

 pigmented cells perfectly hyaline). For maceration, use one- 

 third alcohol ; the aceto-osmic mixture failing to preserve the 

 rods of the pigmented cells. 



739. Ophiuroidea. Should be killed in fresh water if it be 

 desired to avoid rupture of the rays (DE CASTELLARNAU, La 

 Est. Zool. de Napoles, p. 135). 



740. Larvae of Echinodermata. I am greatly obliged to my 

 able friend Dr. BARROIS for kindly writing down for me (for 

 the Traite des Meth. Techn., from which they are translated) 

 the following instructions, which are the outcome of a pro- 

 longed and minute study of the metamorphoses of the Echino- 

 dermata. 



Pluteus. In order to a fruitful study of the metamorphoses 

 of the Echinoidea and Ophiuroidea, it is necessary to obtain 

 preparations that offer the advantages presented by the study 

 of the living larvae ; and especially such as give distinct 

 images of the different organs, and show the calcareous 

 skeleton preserved intact (a point of considerable importance, 

 since this skeleton frequently affords landmarks of the greatest 

 value). These preparations should further possess the fol- 

 lowing points : They should give clear views of the region 

 of formation of the young Echinoderm (which is generally 

 opaque in the living larva). And they should possess suffi- 

 cient stiffness to allow of the larva being turned about in any 

 desired way and placed in any position under the microscope. 



It is not very easy to obtain preparations fulfilling these 

 conditions, on account of the difficulty of obtaining a selective 

 stain whilst preserving the integrity of the calcareous skeleton. 

 The following method is recommended : Pluteus larvae are 

 fixed in a cold saturated solution of corrosive sublimate, in 

 which they remain not more than two or three minutes. They 

 are then washed with water, and brought into dilute Mayer's 

 cochineal. This should be so dilute as to possess a barely 

 perceptible tinge of colour. The objects should remain in 

 the stain for from twelve to twenty-four hours, being carefully 

 watched the while, and removed from the stain at the right 

 moment and mounted in balsam, or, which is frequently 

 better, in oil of cloves or cedar wood. This method is pi-r- 



