260 



ZOOLOGY 



SECT. 



uiinogenital papilla into the cloaca at the base of the cloacal ciECum 

 referred to on p. 258. Many of the cellular elements, such as 

 the blood-corpuscles, are of comparatively large size. There is 

 holoblastic, but unequal, segmentation, as in Ceratodus, followed 

 by a true invagination. A pair of medullary folds are developed, 

 and between them is formed a median solid ectodermal keel in 

 which a neuroccele only appears subsequently. The larva has well- 

 developed external gills. 



The Dipnoi are a very ancient race. The genus Ceratodus 

 itself extends back to the early Mesozoic, and the remains of 

 allied forms {Bi^jtcrus and other genera), are found in Devonian 

 and Carboniferous formations. But if, as is conjectured, the 

 Arthrodira are to be regarded as Dipnoi, then the group dates 

 back as far as the Silurian. The evidence for this conclusion is, 

 however, by no means complete, as our knowledge of the 

 structure of the extinct Fishes in question is necessarily meagre. 



Fig, 915. — Coccosteus decipiens. Side view, restored. A, articulation of head with 

 trunk ; BB, cartilaginous basals of dorsal fin ; DR, cartilaginous radials of dorsal fin ; 

 H, hajmal arch and spine ; MC. mucous canals ; iV, neural arch and spine ; V, median 

 unpaii'cd plate (?) of hinder ventral region ; VB, basals of pelvic fin ; VH, radials of pelvic 

 fin. (From Dean, after Smith Woodward.) 



They had the head and anterior part of both dorsal and ventral 

 surfaces (Fig. 915) protected by bony plates, the system of head- 

 plates being connected with those on the trunk by a well- 

 developed movable joint. The notochord was persistent, with 

 partly calcified neural and hsemal arches, and the cranium was 

 apparently cartilaginous; the mandible was probably autostylic. 

 There were composite cutting dental plates. The pectoral fins are 

 unknown; the rays of the small pelvic {VR) were supported on a 

 flattened plate ( VB). 



With some special features of their own the Dipnoi combine 

 characteristics in which they resemble now one, now another, of 

 the other groups of Fishes, together with a few in which they 

 approach the next class of Vertebrates to be dealt with, viz. the 

 Amphibia. The brain and the heart are quite peculiar : the former 

 in its undivided, or almost undivided, mid-brain ; the latter in its 

 imperfectly divided auricle, and spirally twisted conus. In the 

 limbs the Dipnoi are only closely approached by certain extinct 



