130 THE INVERTEBRATA 



from the latter, their cavity becoming the coelom and their wall the 

 mesothelium. In other cases it arises as solid outgrowths or layers 

 shed off from the wall of the archenteron, and coelomic cavities after- 

 wards appear in it. This happens, for instance, in the tadpole. In yet 

 other cases a single pole cell or teloblast^ as in annelids (Fig. 196) and 

 molluscs, or a group of a few cells, as in arthropods, separate, on each 

 side of the embryo, from the rudiment of the endoderm, and multiply 

 so as to form a band of cells in which coelomic cavities appear. A 

 coelom which arises as a pouch from the archenteron is known as an 

 enter ocoele ; one which arises in a mass of mesothelium is a schizocoele. 



In the lower triploblastic phyla (Platyhelminthes, Nemertea, 

 Nematoda, etc., p. 197) there is no mesothelium. Chaetognatha have 

 no mesenchyme. In most phyla, both kinds of mesoderm develop. ^ 



We must now consider the organs formed by each of the three 

 layers. 



i. Endodermal organs. After giving rise to mesoderm, the archen- 

 teron becomes the rudiment of the alimentary canal. Except in 

 Platyhelminthes, its blastopore is in various ways replaced by two 

 openings,^ so that it has both mouth and anus. Its wall, the endoderm, 

 forms the lining of the alimentary canal, except in those regions, 

 known as fore gut or stomodaeum and hind gut or proctodaeum, which 

 are formed by a tucking-in of the ectoderm at the mouth and anus. 

 The endoderm also gives rise to the various diverticula of the mid gut, 

 such as the liver and other digestive glands, the lungs of vertebrata,etc. 

 A true stomach is an enlargement of the mid gut. 



Digestion was perhaps originally entirely intracellular in the endo- 

 derm cells, and many of the lower animals still have intracellular 

 digestion, though this is usually preceded by an extracellular process 

 which by dissolving certain components of the food enables the re- 

 mainder to be reduced to particles small enough to be taken up by 

 the cells. In the annelids, arthropods (except certain ticks, p. 534), 

 cuttlefishes, and Chordata digestion is entirely extracellular. The 

 enzymes secreted vary with the food : in carnivorous animals such 

 as cephalopods and starfishes they are principally proteases, in feeders 

 on vegetable tissues they are largely carbohydrases, in omnivores such 

 as the crayfish and cockroach and holothurians they are adapted to 

 deal with all classes of food-stuffs. Considering the importance of 

 cellulose both as a potential food-stuff and as cell walls which enclose 

 more valuable foods, it is remarkable that cellulases should be rare 



(PP-435, 559>587)-, 



Both intracellular ingestion and absorption are not always confined 



^ Mesenchyme is scanty in the lower Chordata. 



^ The most primitive way is probably that of Peripatus (p. 319), in which 

 the middle of the blastopore closes and the ends become mouth and anus. 



