ON THE REPRODUCTIVE SYSTEM IN THE HYDROIDA. 4.07 
To this stage Sir John G. Dalyell has given the name of “ planula,” a name, 
however, suggested by a mistaken view of its form, which he compares to a 
Planaria. In this comparison he has probably been led astray by the imper- 
fection of the microscope employed; for the locomotive embryo has no ten- 
dency whatever to a flattened shape, as indicated by the name of ‘“ planula,” 
but is always conical or cylindrical. Instead of ‘ planula,” therefore, one is 
strongly tempted to employ for this form of embryo some term which shall 
not tend to convey a false impression of its figure. The term “ planula,” how- 
ever, has passed into such general use, and has, 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. 
The further progress of the animal, up to that stage in which it has ac- 
quired 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 Hudendriwm 
racemosum, Cay., in which I have satisfactorily followed the various steps. 
After the embryo 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 scen with one end dilated, so as to give a flask- 
shaped form to the embryo. 
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. From the centre of this enlarged base the rest 
of the embryo rises perpendicularly as a little cylindrical or slightly 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 cir- 
cular disc of adhesion. At the same time the embryo becomes enlarged a 
little behind its distal or free extremity by the formation of a slightly pro- 
minent circular ridge, while an exceedingly delicate periderm has been excreted 
as a scarcely perceptible film over its whole surface. 
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 commence as shallow notches at the circum- 
ference, and thence gradually increase in depth until they nearly reach the 
central vertical column. These lobes, like 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 deli- 
cate periderm, 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 Hudendrium has increased in size, and the 
circular ridge has become more pronounced, while the part in front of this 
ridge has in the same proportion become more decidedly marked off from the 
rest of the body, and the periderm has here become more distinct by the partial 
withdrawal from it of the included structures. 
Soon after this the whole circumference of the ridge will be found to have 
extended itself as a circle of about ten short, thick tentacula, 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 in front of the circle of rudimental tentacula 
