Affinity of MoIIusca and Molluscoida. By W. K. Broohs. 139 
favourably criticise the work of this author. On the contrary, the 
aDatomical resemblances which he points out so clearly are worthy 
of the most thoughtful attention, and although they are not homo- 
logical and do not indicate descent, they are excellent illustrations 
of the independent origin of similar structures ; a class of relations 
which has not yet been sufficiently allowed for in the speculations of 
the modern school of zoology, but which seems destined to form, at 
some future time, an important element in the theory of the evolu- 
tion of life. The superiority of the conceptions of Allman becomes 
evident as soon as we contrast them with many which have been 
advanced ; for example, the comparison advocated by a very distin- 
guished naturalist and embryologist between the foot of a Lamelli- 
branch, the tail of Appendicularia, and the placenta of Salpa. 
We come now to the question, If our present knowledge of the 
embryology of the Mollusca and Molluscoida disproves all the old 
ideas of their affinity, does it present anything to replace them ? 
Most of the Gasteropoda are known to pass through a free, loco- 
motive "Yeliger" stage. The veligers of different Gasteropods 
differ considerably in form ; and in some the embryo, at this stage, 
is much less specialized than in others ; but, omitting the compli- 
cations introduced as adaptations to a spiral shell, the veliger of 
such a marine Gasteropod as Astyris may be regarded as present- 
ing the typical form. A veliger may be described as a free-swim- 
ming, bilaterally symmetrical embryo, without a true heart or 
vascular system, or branchias, with the mouth and anus near each 
other on the median line. The digestive organs are suspended in 
the body cavity, and attached to the body-wall at the two external 
apertures, and by the various muscles. The foot is situated between 
these two openings; and the pedal ganglia, which are in most 
veligers the first ganglia to appear, are developed in the region of 
the foot ; that is, between the mouth and the anus. The foot is 
generally supplied with a bunch of setso, which are apparently 
sensory in function. The animal is enclosed in a shell composed 
of two portions ; a large ventral cup, and a neural or pedal oper- 
culum, which is united to the anal margin of the cup at the earliest 
stages, and subsequently becomes separated from it. This shell 
and lid are found in the embryos of those forms where the adult is 
without an operculum, as Crepidula, as well as in those where the 
adult is destitute of a shell, as the Nudibranchs. 
The most characteristic peculiarity of the veliger is the velum. 
This is a large bilaterally symmetrical circlet of cilia, developed 
from the cephalic region of the embryo, and supported, at some dis- 
tance from the body, by a transparent double- walled veil, the cavity 
of which is irregularly divided into large sinuses, in free communi- 
cation with the body cavity. The animal swims, usually near the 
surface of the ocean, by means of the long cilia of the velum, which 
