1863.] 427 



cates the position of the larval mouth. At the same time a small 

 round aperture, merely separated from the posterior margin of the 

 mouth by the last ciliated band, becomes connected with the mouth 

 by a short loop-like canal passing under the band, and fulfils the 

 function of an excreting-orifice. A tuft of long cilia, which have a 

 peculiar undulatory motion, is developed at the posterior extremity of 

 the body. The larva now increases rapidly in size, assuming some- 

 what the form of a kidney bean, the mouth answering in position to 

 the hilum. It swims freely in the water, with a swinging semirota- 

 tory motion, by means of its ciliated bands and posterior tuft of cilia. 



Shortly after the larva has attained its definite independent form, 

 ten minute calcareous spicula make their appearance, imbedded 

 within the external sarcode-layer of the expanded anterior portion of 

 the larva. The ten spicula are arranged in two transverse rings of 

 five, the spicula of the anterior row symmetrically superposed on those 

 of the posterior. By the extension of calcareous network, these spi- 

 cula rapidly expand into ten plates, which at length form a trellis 

 enclosing a dodecahedral space, open above and below, within the 

 anterior portion of the zooid. Simultaneously with the appearance 

 of these plates, a series of from seven to ten calcareous rings form a 

 chain passing from the base of the posterior row of plates backwards, 

 curving slightly to the left of the larval mouth, and ending by abut- 

 ting against the centre of a large cribriform plate, which is rapidly 

 developed close to the posterior extremity of the larva. Delicate 

 sheaves of anastomosing calcareous trabeculse shortly arise within 

 these rings, and the series declares itself as the jointed stem of the 

 pentacrinoid stage, the basal and first interradial plates of the calyx 

 being represented by the already formed casket of calcareous network. 

 The skeleton of the Crinoid is thus completely mapped out within 

 the body of the larva, while the latter still retains its independent 

 form and special organs. 



Within the plates of the calyx of the nascent Crinoid two hemi- 

 spherical or reniform masses may now be detected, one superior, of a 

 yellowish, subsequently of a chocolate colour ; the other inferior, 

 colourless and transparent. The lower hemisphere indicates the 

 permanent alimentary canal of the Crinoid, with its glandular follicle ; 

 the upper mass originates the central ring of the ambulacral system, 

 with its caeca passing to the arms. The body of the Crinoid is, how- 



