IG 



pileus, with a zooid appearing iu the centre of eacli, present a very remarkable and 

 unique appearance. 



Tlie Canal System. 



Owing to the contraction of the spirit-specimens, the mode of growth is Ijy no 

 means obvious, but an examination of various stages, and of serial sections through 

 these, makes the mode of increase in the number of the canals fairly clear. The 

 centre of the stem is occupied l)y a number of large canals whose cavities are about 

 3 mm. in diameter. These do not communicate with one another, but at several 

 points solenia can ))e seen connecting them with the canals in the cortical region 

 whose cavities vary from 2 niui.-l mm. iu diameter. The cortical canals terminate 

 basally in a cul-de-sac, while upwards they increase iu diameter so as to give 

 rise to zooids on the pileus portion. The younger zooids are peripheral, and the 

 whole colony may thus be compared to a bundle of compound racemes, the 

 branches of which are hollow, and where the secondaries and tertiaries fuse to the 

 primaries and grow to an equal length with them, so as to result iu a corymb-like 

 expansion. The eight mesenteries of the zooids are continued downwards almost 

 to the very base of the canals, and at the same time the asulear pair can be clearly 

 distinguished by the characteristic ciliated groove. This is also the case in Sipliono- 

 (jorgia and Lemnalia, while iu some of the Nephthyidse, e.g. Spongodes, only 

 the asulear mesenteries are continued into the canals of the stem. 



Origin of the Zooids. 

 Both the central or primary canals and the cortical or secondary canals give 

 rise to zooids in a remarkable and interesting manner. After attaining a certain 

 height, which is practically uniform for the colony, the walls turn inwards, so that 

 the cavity is thus reduced iu diameter ; and when this is approximately one-half of 

 the original measurement a vertical upgrowth again commences, thus forming a 

 cylindrical cup-shaped projection, homologous to the verruca in the Axifera. When 

 tlie height of this part is about 4 mm. the circumference grows out iuto eight 

 digitiform structures, while the wall again growing inwards fuses with the lower 

 part at the eight indentations, forming a similar numljer of short l)lind tubes. This 

 constitutes what might be termed the verruca propei-. Growth still takes j^lace, 

 and a zooid is the result, consisting of a comparatively long stalk bearing the 

 anthocodia. That this is the mode of growth is clearly demonstrable in the 

 younger colonies, and also in the less advanced jjolyps round the periphery of the 

 older colonies. As the colonies grow in size, the verrucfe also become more com- 

 plicated, the terminal stellate part expanding horizontallv to form an octagonal 

 disc, with the indentation less pronounced, containing eight cavities which corre- 

 spond to those formed by the retractor muscles. To complicate matters still further, 

 towards the centre of the older colonies, verrucse which correspond to the primary 

 canals fuse with the adjoining verrucjte so that the canals are now continuous. 



