ON THE PRINCIPAL OF CEPHALIZATION. 167 



the Tunicata (Series I, T) we have, through continued 

 cephalization, the mouth thrown to the bottom of the sac, 

 or nearer the anterior end, and now the anus terminates 

 behind the mouth, and posteriorly. 



The heart has also followed the intestine in its rotation 

 and becomes anterior, and partially dorsal. The nerve 

 mass is still posterior, and occupies a position between the 

 two openings as in Polyzoa. 



We have commencing in this group, the Tunicata, that 

 erratic bending of intestine, and varied position in its anal 

 termination, that is witnessed higher up in the scale, and 

 though apparently governed by no law, we can yet trace 

 the progressive movements toward a normal condition, by 

 comparing Appendicularia, one of the lowest forms of the 

 Tunicates, and representing 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 termi- 

 nates anteriorly. In Salpa it terminates dorsally, on a line 

 with the mouth, though still anteriorly. In Botryllus it 

 creeps up, and terminates nearer the posterior pole of sac, 

 though still dorsally. We have in this genus, and other 

 compound Ascidians, the excurrent orifices of several in- 

 dividuals coalescing, forming a common cloaca for a com- 

 munity. The dorsal flexure is distinctly seen in Clavel- 

 lina borealis. In these three classes ; namely, Polyzoa, 

 Brachiopoda, and Tunicata, the sac is essentially closed at 

 the anterior end, and consequently the mouth opens to- 

 ward the posterior end, and with few exceptions all are 

 attached by the anterior end. 



This makes a natural division, corresponding to the 

 Molluscoidea of Milne-Edwards, the Anthoid Mollusks of 

 Dana, and a portion of the neural division of Huxley. In 

 the Lamellibranchiata (Series I, L) we have the sac open- 

 ing anteriorly, and the mouth permanently occupying the 

 anterior region, though in the lower forms pointing pos- 

 teriorly, 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 enlargement of the 

 anterior opening is clearly seen, wherein the Gastrochoe- 



