^ — som 



MORPHOLOGY OF HJGHER TYPES 115 



one another requires a knowledge of certain fundamental cavities of the 

 body, the coelenteron of simple animals and the coelom of more complex 

 ones. To these cavities attention is first turned. 



Coelenteron. — In the Ccelenterata (Hydra and its allies) there is 

 found within the body a single cavity (Fig. 60) which functions as a 

 digestive and vascular cavity and is therefore sometimes called a gastro- 

 vascular cavity, but the name ccelenteron is more expressive of its struc- 

 tural relations. 



Coelom. — In many higher invertebrates and in the vertebrates there 

 is a second body cavity bounded by peritoneum lying between the gut 

 and the body wall. This second cavity, the coelom, is well shown in a 

 diagrammatic cross-section of a developing Amphioxus (Fig. 79). In 

 the earthworm and in most other Annel- 

 ida the arrangement of the body is 

 clearly that of a tube within a tube. The 

 wall of the outermost tube is the body 

 wall while the alimentary tract forms the 

 inner tube which is supported by circular 

 partitions, the septa. The cavity (coelom) 

 between the walls of the two tubes is 

 lined with the peritoneum, a layer of 

 flattened epithelium. In the vertebrates 

 the inner tube* is suspended from the 

 inner surface of the outer tube by means ^^^ 



of dorsal mesenteries which are merely Fig. 79.-Cross-section of an em- 

 continuations of the peritoneum from the ^ryo of Amphioxus, showing fornia- 

 •11 IT mu 1 .• <• 1 tion of coelom. c, coelom; dy, diges- 



mid-dorsal Ime. The relation of coelom tive tract; dm, dermatome; ec, ecto- 



to intestine is illustrated in Fig. 80. In derm; en, endoderm; mc, myoccele; 

 ,1, . n ' ji 1- 1- ■,■ J.1 1 tnt, myotome; nd, notochord; nt, 



that figure the Ime hmitmg the coelom ^e^ral tube; som, somatic layer of 



represents the peritoneum. Fig. 80 also mesoderm; spl, splanchnic layer of 

 1 1 -n J. J. XV r i it- X I- -1 j.i_ mesoderm covering the digestive 

 clearly illustrates the fact that while the t^act; vm, ventral mesentery. 



organs of the body really lie outside of 



the coelom they are pushed down into it and are suspended by perito- 

 neum. A true coelom always bears the gonads on its walls and is lined 

 with peritoneum. From the latter feature this cavity is frequently called 

 the peritoneal cavity. 



In the roundworms, of which Ascaris may be taken as an example, 

 a cavity exists between the alimentary tract and the body wall but since 

 it is not bounded by peritoneum this cavity is not considered to be a true 

 coelom. 



Simplest Digestive Apparatus. — Since in the Protozoa and sponges 

 digestion is intracellular no digestive system or digestive organ can be 

 said to exist. The most primitive multicellular digestive organ occurs 

 in the simple coelenterates. The gastrovascular cavity of Hydra may 



