DEVELOPMENT. 71 



with great distinctness in tlic fluid wliicli fills tlie tubular lacunas of the young elongated 

 coenosarc. 



Tiie lower segment, on the other hand, instead of pushing forth from the cut extremity a 

 simple continuation of the coenosarc, develops from this extremity a liydranth.' There is thus 

 manifested in the formative force of the Tuhularia-stem a well-marked polaritij, which is rendered 

 very apparent if a segment be cut out from the centre of the stem. In this case, no nuvttcr in 

 what position the segment may lie, that cud of it which was directed downwards or proxiuially 

 while it formed a part of the uimiutilatcd hydroid will never develop a hydranth, b>it will extend 

 itself as a simple cylindrical prolongation of the coenosarc ; while the upper or distal end, instead 

 of becoming simply elongated, will shape itself into a true hydranth ; and all this though, of 

 course, not the least difference in structure or form, can be detected between the two extremities 

 at the time of section. 



It is further manifest from these facts that, when the hydroid is placed under conditions 

 which allow of perfect freedom of growth, there is no such thing as a stationary extremity, 

 both ends being really growing ends, while there exists in every segment a neutral plane midway 

 between the two ends. 



Hydranth-hnd in the Calyptohlastea. — In the development of the bud, the Campanularian 

 and Sertularian hydroids differ in some important features from those which characterise the 

 process just described. The development may be easily watched in many species, as, for example, 

 in Laomedea flexmsa. We may here (woodcut, fig. 33) see it proceed, in the first place, to the 

 formation of a hollow cylindrical branch («), whose cavity is in free communication with that of 

 the coenosarc, and whose distal extremity ends in a cul-de-sac invested, like the rest of the young 

 branch, by the chitinous perisarc. Up to this point the phenomena are precisely similar to what 

 we have just seen in the Tuhularians ; but now the distal extremity of the branch begins to enlarge, 

 and at the same time continues to coat itself with a chitinous perisarc in the form of a capsule, 

 which acquires increased thickness by successive deposits of new matter to its inner surface, 

 thus contrasting with the much thinner pellicle which forms the temporary capsule in certain 

 Tiibidarians. 



The extremity of the branch [Ij] now presents the shape of an inverted cone, plainly recognisable 

 as the body of the budding hydranth, invested with a strong chitinous covering, which is closely 

 applied over its whole surface, and is continuous below with the perisarc covering the rest of the 

 branch. The interior of the young hydranth is hollowed out into a wide cavity lined by a 

 layer of loose cells — the most internal cells of the endoderm — which are filled with a granular 

 pigment. 



The conical enlargement at the extremity of the branch continues to increase in size (c), and 

 we soon see the soft parts within become contracted towards the proximal end of the cone, where 

 they withdraw themselves from contact with the walls of the chitinous capsule, which had up to 

 this time closely embraced them. At the wide or distal end of the cone they still remain adhe- 

 rent to the capsule for some distance downwards, while at the proximal end itself there is also 

 a distinct but narrow zone of contact and adhesion maintained between the internal soft parts 



' The first observations on this subject are those of Dalyell, who has made numerous experiments 

 on the section of the stem in Tu/mlaiia indivisa, with results similar to those here recorded. (See 

 Dalyell, ' Rare and Remarkable Animals,' vol. i, p. 23.) 



