Chap. II. EMBRYOLOGY OF CLAVA. 223 



After segmenting {Fi(/s. 12 and 1.3, P), the young hydroid is irregularly glolnilar 

 in shape {Fig. 14, P), and appears to be composed of two dissimilar substances, 

 namely, an outer, thick, transparent laj'er, which is about one sixth as thick as the 

 whole body, and a very dark inner mass. Finally, the young assumes an elongated, 

 pear-shaped form {Fig. 15, h^). We have not been able to detect any vibratile 

 cilia upon the planula while it was within the parent. After breaking through 

 the confines of the disk, and entering upon the new relations of a free, planula-like 

 hydroid (PI. XXI. Figs. 10 and 10"), the vibratile cilia may be seen covering the 

 whole surface of the body, like short bristles, in a constant state of agitation. The 

 length of these cilia is about equal to the thickness of the outer wall (/) of the 

 body. The inner wall {Fig. 10", /^), which has not been recognizable till noAV, is 

 a little thicker than the outer one, and most distinct at the narrower end of the 

 planula. After swimming about for a while, the planula settles down upon one 

 end, loses its cilia, and its longer, axis assumes a perpendicular position {Fig. 3). 

 At this early stage a marked difference exists between the respective thicknesses 

 of the outer and inner walls ; the outer (/) is hardly half as thick as the inner 

 one (/'). In form, the young hydroid is jjerfectly cylindrical from the rounded top 

 to the broad base. It retains this form until it has doubled its breadth, and is 

 about six times longer than broad, and has, at least, five or six tentacles {Fig. 4, a). 

 The tentacles originate one after the other, apparently from above downwards, and 

 no two are ever on the same level. After this period, the body begins to broaden 

 above {Figs. 5 and 6), and to assume a club-shaped form, while the number of 

 the tentacles continues to increase. By comparing Figs. 5 and G, it will be seen 

 that there is considerable inequality in the develojDment of the tentacles, the larger 

 of two hydroids may possess fewer of these organs than the other, but those of 

 the latter are much smaller. The contractions of the proboscis, and the wide 

 gaping of the mouth {Fig. 6, g^), sometimes reduce the head to such a degree that 

 the tentacles are brought to nearly the same level, whei-e they appear to be dis- 

 posed in a single circle, as in Hydractinia, &c. This may be observed in older 

 individuals {Fig. 7"), where some of the tentacles are rolled inward to the borders 

 of the widely-gaping mouth (/). The individual here alluded to {Figs. 7 and 7"), 

 although it has only nine tentacles, is yet twice as large as those we have com- 

 pared above. Its upper part is as distinctly marked from the stem, as in the 

 adult. The stem does not yet exhibit the remarkable slender base of the adult 

 (PI. XXI. Fig. 2, d). 



As to the reproduction of the hydroid, by budding from adult forms, we can 

 only say that the upright stem never branches nor produces any other buds, except 

 medusoids, and, therefore, the prostrate stolonic portion is the basis of all increase 

 in the number of individuals of the colony. 



