342 AN ELEMENTARY TEXT-BOOK OF BIOLOGY. 



nerves, &c. The notochord does not extend further forward 

 than the middle of the ventral side of the brain. It is sup- 

 plemented, or more or less replaced by a well-developed 

 endoskeleton consisting of at least a vertebral column, brain- 

 case, and sense-capsules; also supports for limbs when these 

 are present. The blood contains red as well as colourless 

 corpuscles, and it is enclosed in a system of tubes comprising 

 a ventral heart, arteries, veins, and capillaries. There is a 

 lymph-system, communicating with the blood-vessels, and 

 consisting of the large ccelomic body-cavity, smaller lymphatic 

 spaces, lymphatic vessels, and " ductless " glands, of which 

 the most conspicuous is the spleen. The body may be 

 regarded as made up of two tubes, (a) a smaller dorsal tube 

 enclosing brain and spinal cord, (/3) a larger ventral tube 

 (coelom) containing most of the other organs. Two compact 

 kidneys are present, each provided with a special longitudinal 

 duct. Ova in most cases shed into coelom and are carried off 

 by two oviducts. When the mesonephros persists it not only 

 performs excretory functions, but may also serve as a channel 

 for carrying off the sperms. When entirely replaced by a 

 metanephros it is converted into a conducting-apparatus for 

 sperms. The outlets of the intestine, excretory and repro- 

 ductive organs, are situated close together and ventrally. 

 There is a distinct brain, outgrowths from which develop 

 into the retinae of two eyes, each of which also possesses a 

 lens formed as an ectodermic involution, and a muscular iris 

 of mesodermic origin. Two olfactory sacs and two mem- 

 branous labyrinths are present, all of which are developed 

 from ectodermic involutions. 



A. Anamniot a. Embryo without amnion. Allantois, when 

 present, is not an embryonic appendage. 



GROUP I. ICHTHYOPSID A. Vertebrates with epi- 

 dermic exoskeleton slight or absent. Lateral line sense- 

 organs and a median fin present at some period of life. 

 The notochord often persists, and the vertebrae, when 

 present, are without epiphyses. The cranium always pos- 

 sesses extensive tracts of cartilage, there is never a well- 

 developed basisphenoid, and the cranial membrane bones, 

 when present, include a parasphenoid. The gut either ends 



