Bxnfal Society. 297 



PROCEEDINGS OF LEARNED SOCIETIES. 



ROYAL SOCIETY. 



Feb. 5, 1863. — Major-General Sabine, Presidert, in the Chair. 



" On the Embryogeny of Comatula rosacea (Linck)." By Prof. 

 WyviUe Thomson, LL.D., F.R.S.E. &c. 



After briefly abstracting Dr. W. Busch's description of the early 

 stages in the growth of the young of Comatula, the author details 

 his own observations, carried on during the last four years, on the 

 development and subsequent changes of the larva. After complete 

 segmentation of the yelk, a more consistent nucleus appears within 

 the mulberry mass still contained within the vitelline membrane. 

 The external more transparent flocculent portion of the yelk liquefies 

 and is absorbed into this nucleus, which gradually assumes the form 

 of the embryo larva, a granular cylinder contracted at either end and 

 girded with four transverse bands of cilia. This cylinder increases 

 in si^e till it nearly fills the vitelline sac, gradually increasing ia 

 transparency, and vJtimately consisting of deUcately vacuolated sar- 

 code, the external surface transparent and studded with pyriform 

 oil-cells, the inner portion semifluid and slightly granular. 



The vitelline membrane now gives way, and, usually shortly after 

 the escape of the larva into the water, the third ciUated band from the 

 anterior extremity arches forwards at one point ; and in the space thua 

 left between it and the fourth band, a large pyriform depression indi- 

 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-hke canal passing under the band, and fulfils the 

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

 pecuUar 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 semi- 

 rotatory motion, by means of its ciliated bands and posterior tuft 

 of ciha. 



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 trelhs 

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

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



Ann. i^ Mag. N. Hist. Ser.3. To/, xi. 20 



