466 GENERAL CONSIDERATIONS 



where spicules form a skeletal support for the body. In coelenterates 

 and ctenophores the mesoglea furnishes some support. In other inverte- 

 brates generally the only internal support is formed by fibrous connective 

 tissues, though in the case of cephalopods and arthropods cartilage is devel- 

 oped around the central nervous system. The chitinous shell of the 

 squid, though developed from a tegumentary pocket, is actually internal. 

 In the case of the cuttlefish lime is added to the shell, which becomes bony. 

 The Aristotle's lantern of the sea urchin is an internal calcareous skeleton. 



In chordates an internal skeleton is well-developed, represented first 

 by a notochord, which later becomes replaced by a vertebral column, 

 to which many other parts are added. In the lowest vertebrates 

 this endoskeleton is membranous with only a little cartilage in some cases; 

 in elasmobranchs it is cartilaginous; and from the bony fishes onward, 

 more or less bony. 



The axial skeleton presents a biogenetic series in that in the embryog- 

 enies of the highest forms there appears a continuous notochord sur- 

 rounded by membranous sheaths, corresponding to the condition in the 

 hag; this is replaced by a cartilaginous, segmented vertebral column, 

 which corresponds to the condition in sharks. This in turn gives way to 

 bony vertebrae, each consisting of several elements as in the bony 

 fishes, amphibians, and reptiles; and in the final stage, seen in the adults 

 of birds and mammals, each vertebra consists of a single bone. 



In the absence of endoskeletons, exoskeletons are often highly devel- 

 oped among the invertebrates. They are in nearly all cases epithelial 

 in origin and, therefore, belong to the tegumentary system. In the 

 echinoderms, however, the plates are developed from the connective 

 tissue below the epithelium and are, in the strict sense of the word, 

 skeletal. 



475. Digestive System. — The only structure concerned with digestion 

 in protozoans and sponges is the food vacuole. In the coelenterates, 

 ctenophores, and flatworms, however, digestion takes place in a gastro- 

 vascular cavity. Roundworms and all invertebrates above them possess 

 an alimentary canal, which may be divided into three parts known re- 

 spectively as the foregut, mid-gut, and hind-gut. The first of these is 

 derived from the stoniodeum, which is an infolding of the body wall that 

 meets the anterior end of the archenteron and through which that cavity 

 comes to open anteriorly. The foregut is lined with ectoderm. The 

 hind-gut is derived in a similar fashion from an infolding that meets the 

 posterior end of the archenteron, and is called the proctodeum. This 

 region of the alimentary canal is also an ectodermal infolding. The 

 mid-gut, which represents the archenteron, includes all of the rest of the 

 alimentary canal and is lined with entoderm. The external opening of 

 the stomodeum is the mouth opening; that of the proctodeum, the anal 

 opening. There is considerable difference in the extent of the alimentary 



