Chap. VII. DYNAMENA PUMILA. 331 



wel)bed foot, the middle lobe, or the main stem (/^), being slightly in advance, 

 and the two lateral ones {f_P) communicating with it only through their bases 

 (e^). Each lobe is entirely distinct from the others from its base upwards, and 

 besides, it is capped by a thin, chitinous layer [h I?-), which, however, is distinct 

 from that of the next lobe, only at the base {h), where it forms a single coating 

 to the outer surfice of the adjoining walls («^ a.-), Init toward the end {h) fuses 

 with its neighbor, and, finally, with the terminal, general envelop (c). It is 

 impossible to say what is the definite thickness of the walls, because of the great 

 degree of contraction and expansion which they exhibit; but they may, in general 

 terms, be said to be twice as thick as the fully-developed walls in the oldest 

 part of the stem. The cellular elements are the same as in the last phase. 

 At a later stage {Figs. 6 and 6"), in which the hydras (//^) are just upon the point 

 of developing the free terminal portion, the variable thickness in the walls is 

 represented in our figure, especiallj- in the inner wall {e e^). It will be noticed 

 that the outer wall [a) separates from the chitinous investment very early; but 

 that it is a j^ermanent separation, it is not possible to say. The whole surface 

 of the Inul is so loosely connected with its investment, that it is not a difficult 

 matter to detach it by pressure, and force it out through the basal end, if the 

 stem be cut across a short distance below. By this process, we may get a clear 

 and connected view of the sheath {Fiij. 18), and of the position of the partition 

 walls (a b c). In the figure, the broader side of the stem is next the eye, and, 

 in the distance, the narrower side ; so that the inner sides [a) of the partitions 

 face obliquely toward the observer. As a fnrther step in development, the hydrJB 

 push out, from the hitherto consolidated, budding mass, one on each side of the 

 main stem, like the upper limbs of the letter Y {Fig. 15, /) ; each limb bearing 

 its own .sheath (/). When the hydra has completed its longitudinal growth, as 

 in the figure, and is about to develop its tentacles, its apex is obliquely truncate 

 {I b), or asymmetrically roof-like. The walls (a b), at the upper part, are closely 

 pressed against the sheathing calycle (/), and present a pretty uniform thickness 

 in this region ; but below, they are retracted and much thinner ; still the outer 

 wall adheres, by prop-like processes («^), to the calycle. Finally, the hydra is 

 completed by the scalloping of its distal end, the lobes of the scallop eventually 

 elongating into tentacles {Figs. 3 and 12), and the body retracts altogether from 

 the calycle, after having completed the formation of a bivalve-like operculum 

 {Figs. 2, 3, 4, and 4", op). At or about the same time, the semi-partition {Fig. 2, d) 

 is constructed. When the hydra protrudes from its calycle for the first time, it 

 pu.shes aside the operculum {Figs. 4 and 4% oj)), yet the latter may remain adherent 

 for some time, but evidently for no particular purpose. 



The reproductive calycles usually originate just beneath the hydra {Fig. 10"), 



