with the presence of the genital pleurae there is a pair of vascular folds
of the basement membrane proceeding from the dorsal wall of the gut in the
post-branchial portion of the branchio-genital region, and from the dorsal
angles made by the pleural folds with the body-wall in the pharyngeal
region; they pass, in their most fully developed condition, to the free
border of the genital pleurae. These vascular membranes are called the
lateral septa. Since there are many species which do not possess these
genital pleurae, the question arises as to whether their presence or their
absence is the more primitive condition. Without attempting to answer this
question categorically, it may be pointed out that within the limits of the
family (_Ptychoderidae_) which is especially characterized by their
presence there are some species in [v.03 p.0239] which the genital pleurae
are quite obsolete, and yet lateral septa occur (_e.g._ _Ptychodera
ruficollis_), seeming to indicate that the pleural folds have in such cases
been secondarily suppressed.

_Development_.--The development of _Balanoglossus_ takes place according to
two different schemes, known as direct and indirect, correlated with the
occurrence in the group of two kinds of ova, large and small. Direct
development, in which the adult form is achieved without striking
metamorphosis by a gradual succession of stages, seems to be confined to
the family _Balanoglossidae_. The remaining two families of Enteropneusta,
_Ptychoderidae_ and _Spengelidae_, contain species of which probably all
pursue an indirect course of development, culminating in a metamorphosis by
which the adult form is attained. In these cases the larva, called
_Tornaria_, is pelagic and transparent, and possesses a complicated
ciliated seam, the longitudinal ciliated band, often drawn out into
convoluted bays and lappets. In addition to this ciliated band the form of
the _Tornaria_ is quite characteristic and unlike the adult. The Tornaria
larva offers a certain similarity to larvae of Echinoderms (sea-urchins,
star-fishes, and sea-cucumbers), and when first discovered was so
described. It is within the bounds of possibility that _Tornaria_ actually
does indicate a remote affinity on the part of the Enteropneusta to the
Echinoderms, not only on account of its external form, but also by reason
of the possession of a dorsal water-pore communicating with the anterior
body-cavity. In the direct development Bateson