16 



* 



pilous, with a zooid appearing in the centre of each, present a very remarkable and 



unique appearance. 



The Canal Xi/xtcm. 



( hving to the contraction of the spirit-specimens, the mode of growth is by 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 by 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 be seen connecting them with the canals in the cortical region 

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

 basally in a cul-de-sac, while upwards they increase in 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 in 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 asulcar pair can be clearly 

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

 gorgia and Lemnalia, while in some of the Nephthyidse, e.g. Spongodes, only 

 the asulcar 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 in 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 

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

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

 part at the eight indentations, forming a similar number of short blind tubes. This 

 constitutes what might be termed the verruca proper. Growth still takes place, 

 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 polyps round the periphery of the 

 older colonies. As the colonies giow in size, the verrucee also become more com- 

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

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

 spond to those formed l>y the retractor muscles. To complicate matters still further, 

 towards the centre of the older colonies, verruca; which correspond to the primary 

 canals fuse with the adjoining verructu so that the canals arc now continuous. 



