354 



Vertebrata. 



continuous below witli the covering of the yolk sac, is termed the serous 

 membrane. Further, there grows into the cavity between the serous membrane 

 and the amnion, an outgrowth from the hinder part of the gut, consisting of an 

 inner layer of hypoblast and an outer layer of mesoblast. This outgi-owth, 

 the allantois, grows gi-adually into a compressed sac of considerable size, 

 lying between the amnion and the serous coat, and commimicating by a duct 

 with the gut. The allantois is very vascular, and sei-ves partly as a reservoir 

 for the urinary excretions, partly as a respiratory organ. Similar conditions 

 obtain in other Birds, Reptiles, and Mammals ; in the last-mentioned the allantois 

 unites with the serous membrane to f oi-m vascular outgrowths, which grow into 

 the wall of the uterus and serve as nutritive apparatus (placenta).* In Fish and 

 Amphibia these embryonic membranes are wanting. 



Tlie Vertebrata occupy a somewliat isolated position in the Animal 

 Kingdom, and so far no close connection with other phyla has been 

 proved. Of the five chief divisions of the Vertebrata, excluding 

 Amphioxus, Pisces and Amphibia contrast, in many respects [see the 

 summary below), with the Reptilia, Aves, and Mammalia, which are 

 likewise allied in many ways; the first two are united under the 

 heading Anamnia, the others as Amniota. On the other 

 hand, however, the Amphibia display many points in common with 

 the Amniota as opposed to the Fish (c/., the left of the summary). 



Skeleton of limbs not di- C 

 vided into arm, forearm, 

 finger, etc. 



Stratum corneum absent. 



Olfactory organ a pit. 



Eyelids absent. 



Auditory ossicles absent. 



Aiu-icle of heart not divi- 

 ded. 



Skeleton of limbs divided 

 into arm, fore arm, 

 finger, etc. 



Stratum corneum present. 



Olfactory organ with ex- 

 ternal and internal aper- 

 tures. 



Eyelids present. 



Auditory ossicles. 



Auricle of heart divided. 



1. Leptocardii 



2. Pisces 



3. Amphibia J 



4. Reptilia 



6. Aves 



6. Mammalia 



Embryonic membranes ab- 

 sent. 



Ist and 2nd vertebrse not 

 specially developed. 

 ► Grills present at least in the 

 young forms. 



Mesonephros functional kid- 

 ney of adult. 



Venti-icle not divided. 



Embryonic membranes de- 

 veloped. 



1st and 2nd vertebrse de- 

 veloped as atlas and axis. 



Gills absent. 



Mesonephros replaced by 

 metanephros. 



Ventricle completely or 

 incompletely divided. 



Class 1. Leptocardii (lancelets). 



The body is elongate, compressed, and pointed at each end ; along 

 the back and the ventral side of the tail is a fin ; limbs are absent. 

 The skeleton is represented by a well-developed notochord, 

 running the whole length of the body, and pointed anteriorly and 



• A structure similar to the embryonic membranes occurs in the Inseota and 

 several Worms. 



