The lungs are remarkable for the prominent development of trabecular projections and niches, so that their free lumen is much restricted; they have thereby reached a much higher stage than in any other Amphibia or even many Autosauri. The persistence of an arteria sacralis s. caudalis, a vessel absolutely absent in the adult _Rana_, is a primitive feature, and the same applies to the presence of a true first spinal or suboccipital nerve. The skin of the back and belly is supplied by two great branches from the arteria anonyma, one arising proximally, the other distally from the subclavian; herewith is correlated the almost complete absence of the arteria cutanea magna, which as a branch of the ductus pulmo-cutaneus plays such a prominent rôle in the other Anura. Only in _Pipa_, but not in _Xenopus_, is the great cutaneous vein represented by a very small branch. Both these genera possess a much more complicated "diaphragm" than the other Anura, chiefly owing to a special muscle which arises {145}from the anterior end of the ilia and spreads out fan-like to the oesophagus and to the bases of the lungs.[71] This diaphragmatic arrangement is correlated with the great development of the lungs, and is not a primitive but an advanced feature. It is reasonable to suppose that this has caused the reduction of the usual arteria pulmo-cutanea, and that the other two cutaneous arteries have been developed secondarily. The Aglossa are generally considered as the lowest Anura, and only Cope looked upon _Pipa_ and _Xenopus_ as two convergent terminal branches. Beddard came to the conclusion that both are closely related to each other, chiefly on account of their peculiar diaphragmatic arrangement. The whole question has entered upon a new stage since the recent discovery of _Hymenochirus_, which is in many ways intermediate between the two other genera. Moreover, the mid-Tertiary _Palaeobatrachus_ of Europe is undoubtedly related to them, and we conclude now that all these four genera belong to one group with a distribution formerly much wider than Africa and part of South America. But this does not necessarily mean that the Aglossa are in all respects the most primitive group of living Anura. On the contrary, they possess few decidedly primitive characters, namely, the long typical ribs, the presence of the first spinal nerve, the unimportant persistence of the arteria sacralis, and lastly, the possession in the tadpoles of a right and left opercular "spiracle." The absence of the tongue cannot possibly be an archaic feature, considering its universal presence in all the other Amphibia, including the Apoda, and the suggestive circumstance that this organ is least developed in the entirely aquatic members of the Urodela. In fact, thoroughly aquatic creatures, which seize and swallow their prey under water, require no elaborate tongue; and since we know that the Anura must owe their typical formation to terrestrial life, it follows that those