DEVELOPMENT. 89 



this form of embryo some term wliicli shall not tend to convey a false impression of its figure. 

 The term " planula," however, has passed into such general use, and lias, moreover, become so 

 intimately associated with the memory of one to whose admirable and conscientious observations 

 our knowledge of the Hydroida owes so much, that the defects of the term will hardly justify 

 our suppression of it. 



Further BeveJopment from lite Planula to the attainment of the Adult Form. — The further 

 progress of the animal, up to that stage in which it has acquired all the essential features of the 

 adult, admits of being easily traced in many different species. I shall take as a good type of the 

 changes which the ciliated embryo undergoes in this progress the development of Eudendrium 

 ramosum (PL XIII), in which I have satisfactorily followed the various steps. 



After the embryo (PI. XIII, fig. 10) has enjoyed for a period (which probably extends over 

 two or three days) its locomotive existence, it loses its cilia, and with them all power of active 

 locomotion, though still apparently retaining the power of slowly creeping from place to place by 

 the contractility of its body. It may now be occasionally seen with one end dilated, so as to 

 assume a flask-shaped form (fig. 11). 



We next find that the animal has attached itself to some fixed object by the enlarged extremity 

 of its body, which becomes flattened over the surface to which it thus adheres (fig. 12). From 

 the centre of this enlarged base the rest of the embryo rises perpendicularly as a little cylindrical 

 or somewhat clavate hollow column. The base now expands laterally, while, at the same time, it 

 becomes compressed vertically, so as to acquire the condition of a little circular disc of adhesion ; 

 and simultaneously with these changes the embryo becomes enlarged a little behind its distal or 

 free extremity by the formation of a slightly prominent circular ridge, while an exceedingly delicate 

 perisarc has been excreted as a scarcely perceptible film over its whole surface (fig. 13). 



It will next be seen that a remarkable change has taken place in the disc of attachment by 

 the division of this part into lobes separated from one another by radiating fissures, which com- 

 mence as shallow notches at the circumference, and thence gradually increase in depth until they 

 nearly reach the central vertical column (fig. 14). These lobes, hke the rest of the young 

 hydroid, consist of a layer of endoderm enveloped by one of ectoderm, while each con- 

 tains a prolongation from the cavity of the column, and is invested by a delicate perisarc, 

 which may be traced into the bottom of the dividing fissures. The lobes of the disc increase 

 in number by successive dichotomous division, though absolute regularity is not usually 

 maintained. 



In the mean time the young Eudendrium has increased in size, and the circular ridge has 

 become more pronounced, while the part at the distal side of this ridge has in the same proportion 

 become more decidedly marked off from the rest of the body, and the perisarc has here become 

 more distinct by the partial withdrawal from it of the included structures. 



Soon after this the circumference of the ridge will be found to have extended itself as a 

 circle of about ten short, thick tentacles, while at some distance behind these the body is seen to 

 be narrowed into a short, nearly cylindrical stem, springing directly from the centre of the basal 

 disc ; and the more contracted portion which lies at the distal side of the circle of rudimental tentacles 

 is now plainly recognisable as the proboscis or hypostome of the future hydranth. The tentacles 

 now rapidly multiply by the intercalation of others between those already formed (fig. 15). The 

 second set may at first be easily distinguished by their shortness ; but the bases of all seem to be 

 on the same level, and the whole appeal' to constitute a single unintemipted series. The tentacles, 



12 



