MORPHOLOGY OF ANIMALS 



While the brain lies at or near the anterior pole in all 

 Metazoa, the rest of the central nervous system lies on the 

 ventral side of the body in most invertebrates, but on the 

 dorsal side in vertebrates. The heart lies on the dorsal 

 side in many invertebrates (annelids, arthropods) but on 

 the ventral side in vertebrates. The side of the body on 

 which most of the nervous system lies is called the neural 

 side; that on which the heart lies is the haemal side, 



A few Heteraxonia are apparently radially symmetrical 

 (starfish, sea urchins, etc.), but they are really bilateral. 

 Such apparently radial animals develop from bilateral 

 larvae, probably in adaptation to a sedentary life. The 

 internal organs of Heteraxonia are frequently asymmet- 

 rical, thus the heart of man is on the left side and the 

 larger part of the liver on the right, but this asymmetrical 

 condition is derived from a bilateral stage. Similarly some 

 adult animals, such as snails, Amphioxus, flounders, etc., 

 are strikingly asymmetrical but they are derived from bi- 

 lateral embryos. 



Mesoderm and Body Cavities. These develop in the space 

 between the ectoderm and endoderm, that is in the blasto- 

 coel or pri?nary body-cavity (Fig. i, bc^ Fig. 5, ^, in A and 

 B). In the lowest Metazoa the blastocoel is filled with 

 scattered branched cells (mesenchyme), the spaces be- 

 tween cells being parts of the blastocoel (sponges, Cni- 

 daria, Ctenophora, flat- worms). Among higher Metazoa 

 the mesoderm is usually divided into an inner portion 

 lying next to the endoderm and an outer one next to the 

 ectoderm. Between these is a space, the coelom or secon- 

 dary body-cavity (r, Fig. 5, C and D). This is lined by 



C >9 3 



