

MOLLUSCOi'DA. 307 



tend to an elliptical form, but those of larger dimensions are linear, 

 with parallel sides, and curved extremities. The middle of the upper 

 and the whole of the under surface are smooth, the former somewhat 

 convex, occasioned by a border of seventy or eighty, even up to three 

 hundred and fifty, individual polypi, dispersed in a triple row, their 

 number depending entirely on the size of the specimen. Each of 

 these numerous polypi, though an integral portion of the common 

 mass, is a distinct animal, endowed with separate action and sensation. 

 The body rising about a line above a tubular fleshy stem, is crowned by 

 a head, which may be circumscribed by a circle as much in diameter, 

 of a horse-shoe shape, and bordered by a hundred tentacula. Towards 

 one side, the mouth, of singular mechanism, seems to have projecting 

 lips and to open as a valve, which folds up within, conveying the 

 particles which are absorbed to the wide orifice of an intestinal organ, 

 which descends, perhaps, in a convolution below; and returns again, 

 terminating in an excretory canal under the site of the tentacula." 



The inhabitants of the colony are then united in great numbers 

 under one common envelope ; these are longish filaments of the size 

 of a swan's feather, re- 

 minding one of the appear- 

 ance of the silk thread 

 known by embroiderers 

 as chenille. The downy 

 appearance is produced by 

 the collection of tentacnla 

 belonging to this curious 

 swarm. The filamentous 



maSS is the translucent Fig. 122. Cristatella mucedo CCuvier). 



row of cells in which these 



animalcules are lodged, and to which they retreat when disturbed. 



These cells are sometimes free in part, sometimes completely rooted to 



the stems of aquatic plants. The tentacles are of a fine transparent 



glass colour, the body being of a brown colour. Fig. 122 represents 



Cristatella mucedo, which is common both in this country and in 



France. 



Most naturalists have now agreed to place among the 'Bryozoa 

 certain species of animalcules which long remained imperfectly known. 

 Amongst these are the Flustra, the Eschara, and other ascidians. 



x 2 



