a es cae OR ee baler Se Tet thn 2 
on the Principle of Cephalization. 23 
apparently governed by no law, we can yet trace be sity tines 
movements toward a normal condition, by comparing Appendi- 
cularia, one of the lowest forms of the ieee ae repre- 
senting the larval condition of their class. In this form the 
intestine has a ventral flexure, and terminates on the ventral 
side. In Pyrosoma it makes an abrupt bend toward the anterior 
dorsal region, and terminates anteriorly. In Salpa it terminates 
dorsally, on a line with the mouth, though still anteriorly. 
Botryllus it creeps up, and terminates nearer _ posterior pole 
of the sac, though still dorsally. We have in this genus, and 
other compound Ascidians, the excurrent oie of several in- 
—o coalescing, forming a common cloaca for a community. 
e dorsal flexure is oe seen in Giayalliva borealis. In 
these rear classes; namely, Polyzoa, Brachiopoda, and Tuni- 
cata, the sac is essentially closed at the anterior end, and conse- 
quently the mouth opens toward the posterior end, and with 
few exceptions all are attached by the anterior en 
This makes a natural division, corresponding to the Mollus- 
coidea of Milne-Edwards, the Anthoid Mollusks of Dana, and a 
portion of the neural division of Hux xley. In the Lamelli- 
branchiata — I, L) we have the sac opening anteriorly, 
d the mo permanently occupying the anterior region, 
though in ha seni forms pointing posteriorly, and in all cases 
the tentacular lobes pointing in that direction, and the mouth 
bent downward (ventrally), and partially obstructed by the 
anterior adductor, or by the undivided mantle. The gradual 
ry of the eae poset is clearly seen, rea in 
highest class, all this display of structure lies at the anterior 
pole. Advancing from the Polyzoa, by the gradual advance of © 
the mouth, the posterior pole becomes less prominent. ve 
when the sac opens anteriorly, as in the Lamellibranchiat. 
