265 



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 measure 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 

 measure 2 mm to 1 mm in diameter. The cortical canals terminate ba- 

 sally 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 peri- 

 pheral 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 asuclar pair can be clearly 

 distinguished by the caracteristic ciliated groove (figs. 2 and 3). This is 

 also the case in Sipkonogorgia (Köll.) and Lemnalia^ (Bourne) while in 

 some of the Nephthyidae 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 gi\e rise to zooids in a remarkable and interesting manner. After 



Yiçr. 4. Fiçf. 5 Fig. 6. 



Fig. 7. 



Fig. 4. Young colony (4/j N.S.), showing first stage in the development of the Verrucae. 



Fig. 5. Surface view of a verruca-disc, the zooid being introverted (x 5). 



Fig. 6. Vertical section through a colony showing the relations of the parts (4/,-, N. S.) 



Fig. 7. Slightly enlarged polyp and stem canal showing the method of growth. 



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 (fig. 7), 

 a vertical upgrowth again commences, thus forming a cyhndrical 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 blind short tubes. This constitutes what might be termed the verruca 



