772 
ZOOLOGICAL LITERATURE. 
become united along their sides and at their summit so as to form a closed 
chamber, within which the gonangia with their contents are developed. 
Under the head of Development ” the author treats of non-sexual repro- 
duction and of true generation. Under the fomier heiul he describes the de- 
velopment of the bud in the various zooids of the trophosome and the gono- 
some, and draws attention to a phenomenon which he regards as indicative 
of a true polarity in the organic forces of the hydroida. This shows itself 
in the fact of a segment cut from the centre of a 2\ihularia-s,tQm developing 
a new polypite only from that end which was originally situated distally, 
while it developes a simple continuation of the stem from the proximal end, 
and this no matter in what position the segment may lie after separation. 
In the development of the medusa of Corymorpha nutans he shows that 
the four radiating canals extend themselves as tubular processes round the 
summit of the primordial bud in the mammary tentacles, but carry with 
them at the same time an extension of the ectoderm of the bud, which unites 
them by a web-like membrane, which ultimately becomes the umbrella. 
The distal extremities of the processes become enlarged, approach one 
another, unite, and allow their cavities to intercommunicate. They then 
retreat from one another, but continue in connexion by a tubular elongation 
of the original points of union ; this becomes the circular canal. 
The development of the embryo in Laomeclea Jtexuosa is described as a 
typical example of the mode in which this phenomenon takes place in the 
great majority of the hydroids. The disappearance of the germinal vesicle 
and spot is the precursor of the segmentation by which tlie vitellus becomes 
at last broken \ip into a multitude of protoplasmic masses, each with its 
nucleus. The most superficial of these masses arrange themselves into a 
distinct structure, enveloping all the more internal parts, which are soon 
after seen to consist of an aggregation of cells, each with an endogenous 
brood. The embryo now becomes elongated by bending over on itself. A 
cavity is formed in its axis, apparently by the liquefaction of the most 
deeply seated cells, and it now escapes from the gonophore as a ciliated planula. 
In Tubularia the process is different. INIasses without any distinguishable 
nuclei are detached in succession from a voluminous plasma which enve- 
lopes the central spadix of the gonophore, and these, without undergoing 
any segmentation, become gradually transformed into a polypoid embryo, 
termed “actinula” by the author. Every hydroid, if we except such as 
may bo proved to pass to the medusal condition directly from the egg, 
commences its free existence either as a planula or an actimda. 
The view taken by Claparede of the development of Tuhularia, in which 
he compares the actinula to a medusa, whose manubrium becomes the stem 
of the future 'Tubularia, and which developes the futur e mouth on the sum- 
mit of its umbrella, is regarded by the author as based on an error of in- 
terpretation. 
The various facts which have been from time to time recorded as evidence 
of the direct development of the medusa from the egg are carefully exa- 
mined, and it is concluded that only in a single instance can these facts 
be regarded as afibrding solid proof of such direct development. This so- 
litary instance we find in an observation of Claparede, wlio has seen the 
manubrium of a Lizzia loaded with eggs, witliin which young medusa 
might be witnessed in various stages of development. 
