HTDROZOA. 65 



of outer and inner layers, enclosing- a central 

 cavity {fig- 15, c). Soon it assumes a somewhat 

 pyriform fignre, enlarging at one extremity, in the 

 centre of which a depression is observable. Next, 

 the narrower end attaches itself to some sub- 

 marine object, while the depression at the oppo- 

 site extremity, becoming deeper and deeper, at 

 length communicates with the interior cavity. 

 Thus a mouth is formed, around which may be 

 seen four small j^rotuberances, the rudiments of 

 tentacula (ti). In the interspaces of these four 

 new tentacles arise ; others, in quick succession, 

 make their appearance, until a circlet of numerous 

 filiform appendages, containing thread-cells, sur- 

 rounds tlie distal margin of the " Hydra tuba," 

 as the young organism, at this stage of its career, 

 has been termed by Sir J. G-. Dalyell (e and/). 

 The mouth, m the meantime, from being a mere 

 quadrilateral orifice, grows and lengthens it?elf so 

 as to constitute a true polypite, occupying the axis 

 of the inverted umbrella, or disc, which supports 

 the marginal tentacles. A continuous, wide, open 

 space occupies the whole interior of the umbrella 

 and polypite, whose relations to the rest of the 

 organism, and, indeed, the whole structure of 

 Hydra-tuba, closely resemble what may be seen 

 in Lucernavla. Externally, it presents a delicate, 

 translucent aspect, and in height averages some 3 

 of an inch. But though dissimilar to Hydra in 

 organisation and want of locomotive capacity, the 

 Hydra-tuba recalls to mind its fresh-water con- 

 gener, first, in its remarkable reparative powers ; 

 and, secondly, in the extent to which it multiplies 

 by gemmation. Not merely do buds arise from 

 the sides of the body, but, in addition, creeping 

 r 



