260 DR J. H. ASH WORTH AND DR JAMES RITCHIE ON THE 



conditions by the formation of thin transverse partitions of chitin barring the 

 lumen of the stolon at certain points. In several cases barrier after barrier had been 

 successively formed by the ccenosarc, which had been forced to retreat stage by stage 

 within the stolon (PI. VIII, fig. 14). Such chitinoid barriers are formed by a plug of 

 coenosarc (fig. 15), composed of a circular plate of thick ectoderm and of solid 

 endoderm, similar in general form and structure to the " Deckenplatte," which 

 secretes the operculum in many species of calyptoblastic hydroids. It is clear, 

 therefore, that, in addition to serving as a hold-fast for the colony, the stolon 

 performs an important function during time of stress in retaining and protecting 

 from harmful environmental influences sufficient ccenosarc to give rise, when con- 

 ditions have again become favourable for normal existence, to new series of nutritive 

 and reproductive individuals. 



Hydrocaulus and Hydranth (PI. VI). 



The smallest hydranths observed on the stolon are sessile, are about '2-'3 mm. in 

 length, and have already developed six slender tentacles (fig. l). The stem of the 

 polyp gradually elongates, and not long after the polyp has attained a length of 

 about a millimetre a branch usually appears, others being formed later. The largest 

 living stems observed are 2*8 mm. long and bear four individuals. There are, how- 

 ever, on two of the colonies dead stems 4-5 mm. long, which bore seven to eight 

 individuals. The branches arise at intervals along the stem and lie at acute angles 

 to the main stem, to which they are more or less parallel. It is rare to find the 

 branches subdivided; this occurs only in a very few of the basal branches of the 

 largest stems. Such branches bore two, and in one case three, hydranths, one of 

 which was small — a blastostyle-bud or a very young hydranth. Examination of 

 hydranths of various ages gives the impression that there are longer and shorter 

 tentacles alternating, and that the longer ones in life pointed distally while the shorter 

 ones were directed more or less horizontally. It is, however, difficult to make certain 

 of this in preserved specimens in which the tentacles exhibit various phases of con- 

 traction. When well extended the longer tentacles slightly exceed '5 mm. in length. 



Towards the base of the stem or hydrocaulus the perisarc, which as usual is 

 laminate, is 3-4 /x thick, and of a brownish-yellow colour. Here the covering of 

 extraneous debris is often meagre, in strong contrast to the dense coating present at 

 the distal end of the stem at the base of the hydranth body. In its distal portion 

 the perisarc is less definite in outline, its laminae more feebly marked, and its colour 

 gradually pales until the portion surrounding the base of the hydranth is represented 

 by a thin hyaline substance the outline of which is merely indicated by the presence 

 of adherent foreign particles. 



The hydrocaulus and hydranth have been examined in several series of longi- 

 tudinal sections. As the two cell-layers do not appear to present special features, we 

 do not describe their histology in detail. The ectoderm of the hydrocaulus lies in 



