Chap, ix] PEDICELLARI&. 297 



in and project from the integument. In these 

 Echinoderms, where the defensive powers of the 

 skeleton are slight or lost altogether, we again observe 

 that the creature is prone to acts of self -mutilation, 

 not unfrequently ejecting, when attacked, the whole 

 of its viscera ; these are in time repaired, if the 

 animal is left to recover. 



In the Echinoidea and Asteroidea a number 

 of the spines are not unfrequeiitly converted into 

 stalked or sessile snapping-like organs, the podi- 

 cellariae, as they were called by those who be- 

 lieved them to be independent and parasitic animals ; 

 the sessile pedicellarise are bivalve \ the stalked 

 have three or four valves ; they are supported 

 by the calcareous reticular tissue which is so 

 characteristic of the hard parts of Echinoderms, 

 and are moved by muscles. Their chief function 

 appears to be that of holding on to objects that come 

 into contact with them, or to such supports for the 

 progression of the animal as waving fronds of sea- 

 weed, until the suckers are able to be brought into 

 relation with the object ; it has been observed that 

 their prehensile power only lasts for about two 

 minutes. In some cases it is probable that some of 

 the pedicellarise are used for the purpose of cleansing 

 the neighbouring spines of foreign or f?ecal material ; 

 but, if we are to judge from the great differences 

 which obtain in their number, and their complete, 

 or almost complete absence from some species, the 

 close allies of which have a large number, we are 

 led to believe that their function is not important, 

 and that they have an inverse ratio of development 

 to the size and number of the spines proper (Fig. 

 121 ; c, D). 



The Artliropoda are as definitely characterised 

 by the development of a chitinous, as are the Echino- 

 dermata by that of a calcareous skeleton ; this is 



