574 



PENTACRINOID LARVA OF ANTEDON. 



young of Antedon is attached by a stalk to a fixed base, as 

 shown in Fig. 299 ; but when it has arrived at a certain stag? 

 of development, it drops-off from this like a fruit from its 

 stalk ; and the animal is thenceforth free to move through 

 the Ocean-waters it inhabits. It can swim with considerable 



activity ; but it exerts 

 Fig. 299. this power chiefly to gain 



a suitable place for attach- 

 ing itself by means of the 

 jointed prehensile cirrhi 

 put forth from the under 

 side of the central disk 

 (Fig. 298), so that, not- 

 withstanding its locomo- 

 tive power, it is nearly 

 as stationary in its free 

 adult condition, as it 

 is in its earlier Pen- 

 tacrinoid stage. The Pen- 

 tacrinoid Larva, — first 

 discovered by Mr. 3. V. 

 Thomson, of Cork, in 1823, 

 but originally supposed by 

 him to be a permanently - 

 attached Crinoid, — forms 

 a most beautiful object 

 for the lower powers of 

 the Microscope, when well 

 preserved in fluid, and 

 viewed by a strong inci- 

 dent light (Plate xxi., 

 fig. 3); and a series of 

 specimens in different 

 stages of development 

 shows most curious modi- 

 fications in the form and 

 arrangement of the various 

 component pieces of its Calcareous skeleton. In its earliest stage 

 (Fig. 299, a), the body is enclosed in a Calyx composed of two 

 circles of plates ; namely, five basals, forming a sort of pyramid 

 whose apex points downwards, and is attached to the highest joint 

 of the Stem ; and five orals superposed on these, forming when 

 closed a like pyramid whose apex points upwards, but usually 

 separating to give passage to the cirrhi, of which a circlet surrounds 

 the mouth. In this condition there is no rudiment of arms. In 

 the more advanced stage shown at b, the Arms have begun to make 

 their appearance ; and the skeleton, when carefully examined, is 

 found to consist of the following pieces, as shown in Plate xxi., 



Crinoid Larva of Antedon :— a, b, c, 

 successive stages of development. 



