ON THE STliUOTURE OF HYDBACTINIA ECIIINATA. 87 



The lower surface of the perisarcal tubes is thicker than the 

 upper surface; this and the side walls are frequently thickened 

 into vertical^ conical spinules, and form the foundation of the 

 characteristic skeleton of older parts of the colony. The thin 

 upper surface has no chitinous thickenings^ and usually stains 

 more readily than the rest of the perisarc. As the coenosarcal 

 tubes continue to grow they branch repeatedly and anastomose 

 with one another until the tubular character of the chitinous 

 investment is obliterated ; in this way a sheet of coenosarc, 

 enclosed between two layers of chitin, is formed. 



When two such tubes anastomose, it is probable that part 

 of the internal lining of ectoderm in each tube which lies 

 nearest to the region of contact absorbs the chitinous wall 

 along this region ; consequently the two tubes are here placed 

 in communication with each other, and the ectoderm of the 

 upper half of one tube becomes continuous with the ectoderm 

 of the upper half of the other tube, while the ectoderm of the 

 lower half of the one becomes continuous with the ectoderm 

 of the lower half of the other. Hence arises the differentia- 

 tion of two ectodermal layers so characteristic of the maturer 

 regions of the colony (PI. 1, figs. 1 and 4). 



The endoderm retains its tubular character. 



The chitinous spinules along the side walls of anastomosing 

 tubes do not all become absorbed, but many remain as part of 

 the permanent skeleton, and are attached at their tips to the 

 thin upper membrane of chitin (PL 1, fig. 1, a, c, b). 



As the colony grows this membrane gradually weakens, and 

 finally is lost (PI. 1, fig. 1, b) ; the coenosarc then grows up 

 over the tips of the spinules, and so becomes a continuous 

 expansion over the surface of the skeleton. 



Here and there are empty chitinous tubes from which the 

 protoplasm seems to have withdrawn itself; frequently all that 

 remain of these tubes are the small chitinous spinules. 



At intervals a small polyp branches from this growing 



region, breaking through the upper chitin, which does not 



extend over the polyp above its base. In some of the colonies 



I examined I found many '^ tentacular polyps'' in this region. 



^ liyj. h^. /h. fin .W)V>«- U..(^ 1 



