WYVILLE THOMSON, ON SVNAPTA INH^RENS. 137 



its outer surface, so that at this stage the embryo has the 

 special character of the coelenterate type. Very shortly, 

 however, there is a softening and absorption of the sarcode 

 immediately round the canal, which then begins to exhibit, 

 though for some time only feebly, its characteristic vermicular 

 motion. 



The general development of the young, and the differen- 

 tiation of the various organs, takes place from this point 

 steaddy, but in a tank in the house extremely slowly. I have 

 now before me (February 10th, 1862) one of the original 

 brood still living, 1*9 mm. in length, and with only seven 

 tentacles. The want of perfect aeration, and the insufficient 

 supply of organic matter in the water are undoubtedly among 

 the causes of this slow progress. I have every reason to be- 

 lieve, however, from the analogy of other cases of develop- 

 ment, that the stages of the process are the same in the tank, 

 and when the animal is free. A body-space gradually be- 

 comes more and more perceptible between the alimentary 

 canal and the body wall, while the latter assumes the form 

 of a consistent membrane of two layers — an outer, thick, 

 structureless and transparent ; and an inner, delicately stri- 

 ated transversely, and highly contractile. The five tentacles 

 become more definite in form, a waved lateral outline in- 

 dicating the origin of the pinnules ; groups of the calcareous 

 fibulae and minute miliary granules accumulate in the sarcode 

 layer of the tentacles ; and usually three anchors, with their 

 anchor-plates, make their appearance simultaneously at irre- 

 gular points on the surface of the body. 



The first indication of an " anchor" is the trend, a delicate 

 calcareous style (PI. VI, fig. 3, a) , hollow, and pointed at both 

 ends. By addition of fine calcareous laminae to the surface, 

 the style becomes blunt at one end, a short transverse crown 

 is gradually formed {b), which lengthens (c), and each arm 

 curves slightly inwards towards the shank {d) . When the 

 form of the anchor has been thus far indicated, the ring end 

 of the rod becomes slightly dilated, and a short, blunt, cal- 

 careous spicula may be detected lying across the centre of the 

 shank and beneath it. This spicula bifurcates at either end, 

 and this process of dichotomous division goes on (e, /, g, h) 

 till the anchor-plate is gradually mapped out of full size, but 

 the network is as yet smooth and extremely delicate (fig. 4). 



While the extension of the network is going on, the old 

 calcareous laminae seem to be absorbed within, as new layers 

 are applied without, for the network appears always to form 

 a continuous tubing. Afterwards the tubes are much thick- 



