STRUCTURE OF THE VERTEBRATES 27 



"primitive" in relation to Amphioxus. The group is primitive 

 because it is fundamentally similar to the earliest ancestors of 

 the phylum. The young Amphioxus is generalized because its 

 characters apply to the chordates as a whole. However, the 

 group has many specializations, or characters which have been 

 added to serve a particular function. The student has but to 

 consider the enormous length of time since the origin of the first 

 Amphioxi to understand this. The constant mutations which 

 occur in all animals; constantly changing environmental condi- 

 tions which place new hazards upon existence; and the destruc- 

 tion of the least adapted types due to natural selection, explain 

 why a group rarely exists for long periods without adding spe- 

 cializations which adapt it for particular conditions. 



Specializations of Amphioxus. An incision in the ventral 

 body wall of a frog or other vertebrate would open the coelomic 

 cavity; but a similar incision in the body of Amphioxus would 

 open an ectoderm lined cavity, the atrium, which serves to pro- 

 tect the gills of the burrowing animal. This cavity is closed at 

 the anterior end and opens posteriorly through the atriopore. 

 Therefore, water entering the mouth of the adult passes through 

 the gills into the atrium, and out through the atriopore. In this 

 way the circulation of water is not hindered by the surrounding 

 sand. Its development is illustrated by the drawings. The atrial 

 folds of the larva begin in the anterior dorsal region and pass 

 posteriorly and ventrally until they nearly meet anterior to the 

 anus. These continuous folds then grow ventrally, being covered 

 on both sides by ectoderm. Eventually they meet along the mid- 

 ventral line, fuse for their length, and develop the new cavity. 

 The metapleural folds are simply longitudinal folds of skin 

 along the lateral walls of the atrial walls. 



The coelomic cavity of the pharyngeal region is also highly 

 specialized. This is due to (1) the increased size of the 

 atrium, and (2) more especially to the great number of gills. 

 The body of a vertebrate can be compared to a room with its 

 inner layer of plaster (the endoderm), the outer layer of brick 

 (the ectoderm), and the cavity between (the coelomic cavity). 

 If the windows are compared with the gill slits, it will readily 

 be seen that every window destroys a certain portion of the wall 



