Apart from the distinctive characters of the six "classes" into which the Craniata are divided, two or three of these classes may possess important structural features in common by which they are distinguished from others. Thus, Cyclostomata, Fishes and Amphibia agree with one another, and differ from all the remaining groups in breathing by gills and in possessing lateral line sensory organs during part, or the whole, of life. Their embryos have no investing amnion, neither does the sac-like outgrowth from the hind-gut, which is known as the allantois, if present at all, ever extend beyond the coelom to form an embryonic investment or to act as a primitive breathing organ. Hence, therefore, the terms Ichthyopsida, Anamniota, and Anallantoidea have been applied to these three classes. Similarly, the term Sauropsida, as applied to Reptiles and Birds, is a convenient means of giving expression to the fact that, underlying the most striking diversity of outward form and habits, there is a community of inward structure which justifies the conclusion that these animals are more closely related to one another than either group is to any other class of Craniates. And again, the application of the terms Agnathostomata and Gnathostomata brings into sharp relief the fundamental distinction between the Cyclostomata and all the remaining groups of Craniata which is {146}only partially illustrated by the presence or absence of biting jaws. In a general and popular sense the Cyclostomata are usually regarded as "Fishes," but this usage rests on no better foundation than a certain agreement between the Cyclostomata and the true Fishes in outward form and habits, and in their method of respiration by gills. On the other hand, it has been maintained that the distinctive features of the Cyclostomata are of sufficient importance not merely to separate them from the true Fishes, but possibly even (as is to some extent expressed by the use of the terms Agnathostomata and Gnathostomata) to warrant their elevation to a group equal in taxonomic value to all the remaining living Craniata taken collectively. The organisms included in the Cyclostomata, the Lampreys, and especially the Hag-Fishes, exhibit in many respects an extremely low grade of Craniate structure; but how far the simplicity or archaic nature of some of their organs is primitive, or has been acquired through degeneration, it is difficult, and is sometimes impossible, to determine with any degree of satisfaction. In other respects, such as the presence of a rasping "tongue," it is obvious that the Cyclostomata have attained a high degree of specialisation. As one of several illustrations which might be given of difficulties of this kind, it may be mentioned that it is by no means certain that the Cyclostomata are not the degenerate descendants of primitive but now extinct Gnathostomata. At all events the presence of paired cartilages in the skull of the Lamprey, which, with some show of