70 MORPHOLOGY. 



in the space between this lireach and the base of the hydranth, the npper end of the stem shps out 

 of its ectoderm, carrying the hydranth with it, and leaving behind it the empty ectoderm as a 

 thin, collapsed, membranous sheath, surrounded by the perisarc, which here exists as a delicate 

 loose pellicle (c). 



The hydranth thus detached falls to the ground, where it retains for some time its vitality, 

 the gonophores which still hang from it discharging such of their contents as had not escaped 

 before the decapitation. 



In the mean time the wound which had been formed in the ccenosarc by the detach- 

 ment of the hydranth heals over, and tlie truncated end of the ccenosarc becomes 

 closed. 



Two slight constrictions, one a little behind the other, are next seen (B) to take place in the 

 ccenosarc at a short distance from the decapitated extremity, while the peculiar tubular lacunae 

 which exist in the ccenosarc of the Tuhidaria, and which had hitherto extended as separate canals 

 through the whole ccenosarc of the stem, now coalesce in front of the anterior constriction (/), where 

 they form a single cavity by the breaking down of the partitions of endoderm which had upon 

 this time separated them from one another. 



A girdle of minute tubercles (<?) may next be seen budding forth from the co-nosarc, at the 

 site of the posterior constriction. These soon become extended into tentacles (C e), which embrace 

 the distal part of the ccenosarc. 



In the next stage a similar zone of tubercles (/), becoming, like the others, elon- 

 gated into tentacles, shows itself close below the anterior constriction ; and there are 

 thus established the two sets of tentacles, the proximal and distal ones of the new 

 hydranth. 



By the elongation of the ccenosarc from below, the new hydranth is gradually lifted up out 

 of the tube of the perisarc, when the tentacles, having room to expand, immediately fall into 

 their normal position, while the rudimental clusters of gonophores may be seen as minute 

 lobulated elevations between the anterior and posterior series of tentacles, and the free extremity 

 of the hydranth has by this time become perforated by a mouth. 



The hydranth now increases in size, raised higher and higher on the elongating ccenosarc, 

 which clothes itself with a perisarc as it lengthens, and the hydranth with its clusters of gono- 

 phores, having finally attained complete maturity, is then in its turn cast off, to be succeeded in an 

 entirely similar way by a new one. 



The formation of successive hydrauths is always accompanied by a periodical elongation of 

 the stem, and this is indicated by annular markings of the periderm separated by rather wide 

 intervals, each interval corresponding to a single decapitation and renewal. 



From the above description it will be seen that the formation of successive polypites is 

 not so much a process of ordinary budding, as a true metamorphosis of the decapitated extremity 

 of the ccenosarc. 



Polarity of the Hydroid. — In connection with the phenomena now described, those which 

 accompany the artificial section of the stem deserve special notice. When the stem is cut across, 

 the ccenosarc of the upper segment soon heals over at the place of section, the tubular lacunae 

 become again closed, and the ccenosarc now begins to grow downwards through the cut extremity 

 of the periderm, presenting the same lacunar structure as in the older portions, and excreting 

 upon its surface a very delicate perisarc. The well-known cyclotic currents may generally be seen 



