CHARACTERISTICS OF THE DIGESTIVE APPARATUS. 



205 



is bordered by cilia) is, therefore, the first point of specialization of 

 the digestive tract. No digestive tube is, however, yet present, but the 

 orifice in the walls of such an animalcule communicates directly with a 

 central body-cavity, and the orifice of entrance and exit of the food is 

 the same (Fig. 56). 



In the hydra there is a definite oral aperture, or mouth, leading to 

 a permanent body cavity, and this opening serves also for the inlet of 

 food and the outlet of the undigestible residue. The hydra has, how- 

 ever, advanced a step in specialization, since it is provided with definite 

 prehensile organs (tentacles) for the seizure of food (Fig. 57). 



There are apparently, however, no true digestive secreting organs in 

 the hydra, since the internal body cavity appears to be strictly the same 

 as the external surface. Hydras have even been everted, so as to make 



Fig. 57.— Hydrozoa. (Jeffrey-Bell. ) 

 A, hydra viridis attached to duck-weed ; B, a single specimen 



magnified ; C, hydra in diagrammatic section. 



E C, ectoderm ; E N, endoderm ; M, mouth ; B C, enteric 



cavity ; T, tentacles. 



Fie. .58.— Flttstka. Diagrammatic 

 Section of a Single Cell of 

 Flustra, ok "Sea-Mat" (greatly 

 magnified). ( Wilson.) 



A A, ciliated tentacles; B, mouth ; C, gullet; 

 D, stomach ; E, intestine ; F, anus ; G, nervous 

 ganglion ; H H, general body cavity ; I, testis : K, 

 ovary ; L, ectoeyst, or outer membrane of body ; 

 M, endocyst, or inner membrane of body ; N, re-, 

 tractor muscle, by the action of which the animal 

 can withdraw the ciliated tentacles. 



the original outside surface the lining surface of the digestive cavity, and 

 digestion was still quite as efficiently performed. Similar types of digest- 

 ive organs are seen in sponges and in the jelly-fish. In the latter, 

 however, a step still further has been made in the development of chan- 

 nels for the distribution of the nutritive substances. 



A true alimentary canal should have two openings, one for the 

 ingress of food, the other for the egress of excreta. The simplest form 

 of such an organ is seen in the flustra, or sea-mat, and in the sea-urchins 

 (Fig. 58.) In these organisms both the mouth and the vent develop con- 

 stricting muscles. In the animals of which the worm is the type (Figs. 

 59 and 60), the digestive tract is either a straight tube running from one 

 .end of the body to the other, or it may be divided into little pouches or 

 saccules, as in the leech (Fig. 61). 



