Pedicellarice in the Echinidae. 109 



bouring spines, and is finally carried off by the ordinary cur- 

 rents of the water in which the Echinus lives. 



A similar process may be observed with the greatest ease 

 to be carried out by Astropecten ; and this I have been 

 able to verify many times by placing a specimen of the 

 common A. aurantiacus in a large flat vessel, convenient for 

 observation, and, when covered with sea- water, sprinkling some 

 fine sand and mud over its dorsal area. In the course of a 

 short time most of this will have been carried away by the 

 action of the paxillas and by the lateral papillated grooves, 

 whilst such particles as have resisted this operation will be 

 found enveloped in a glairy pellicle, which is gradually and by 

 very slight motion drawn into a narrow band extending over 

 the median line of each ray. This is then disengaged from 

 the surface entirely, and is finally cast off by the slightest 

 movement the starfish may make. 



Without some such process it would be difficult to see how 

 so intricate a structure as the paxillary area could be kept 

 free from the collection of dirt and foreign matter, as the 

 interspaces under the extended spinelets of the paxillse form 

 what would naturally become a receptacle for such accumu- 

 lation. 



Without entering further into detail on the present occa- 

 sion as to the source of the mucous discharge in Astropecten 

 (as I hope shortly to communicate some observations upon 

 that point), I may say that I feel little hesitation, after a 

 careful study of the facts above recorded, in regarding this 

 as a strictly homologous case with that of Sphcerechinus, and 

 valuable for our present purpose in exemplifying the manner 

 in which the operation is effected in that Echinoderm. 



Relations, Immature Stages, and probable Origin of the 

 Saccular Organ of the Stem. — At present I am unable to 

 offer any satisfactory reason why there should be the two 

 distinct sources of mucous secretion in Sphozrechinus granu- 

 lans, viz. from the sacs of the valves of the head and from 

 the sacs upon the stem. Very probably the secretion may be 

 of different composition from the two sets of glands. Indeed 

 this supposition would seem to be confirmed by the difference 

 in their histological details, as well as by the frequent (che- 

 mical?) change which is found to take place in the sacculi of 

 the stem and the deposition of crystalline matter therein 

 during preparation, a state of things which I have never met 

 with in the sacs of the head of the pedicellarias. 



At first sight it may appear anomalous that such a well- 

 marked peculiarity as the presence of the stem-sacculi should 

 apparently be possessed only, so far as my present knowledge 



