THE ORGANS. 47 



affect the question of its origin. In other forms (Annulata) these 

 organs, having a tubular form, assist in the generative functions, by 

 serving as ducts for the generative products. The recurrence of this 

 function for a portion of the primitive excretory apparatus (primi- 

 tive kidney, archinephron) of the Vertebrata might be explained as 

 an inheritance from a lower stage. How far such a view is justified 

 is still uncertain. In any case a genetic point of contact between 

 the primordial kidney of Vertebrata and the renal tubes of lower 

 organisms, can only be looked for where the apparatus is, as in the 

 Vertebrata, single on each side of the body. 



h) Alimentary Canal. 

 § 41. 



The ingestion of nutrient matter into the body is, in some of the 

 lowest organisms, effected by endosmotic processes, in which the 

 surface of the body takes the principal share. In others solid nutri- 

 ment is ingested, the soft protoplasm sending out pseudopodia, and 

 embracing the nutrient matter which happens to come into the 

 neighbourhood (Rhizopoda). The formation of a definite part of 

 the surface of the body, serving for the ingestion of nutriment, is 

 really a step towards organological differentiation (Infusoria) ; but 

 this does not constitute an alimentary canal, which does not appear 

 as a separate organ till the body is differentiated into cell-layers. 

 The cell-layers, when they do appear — an inner and an outer — pass 

 into one another at the margin of the orifice of entrance. 



The inner layer, or endoderm, lining a space open to the 

 exterior, forms the wall of a digestive cavity. In the simplest 

 form, represented by the Gastrula, the endoderm is the sole wall of 

 the primitive enteric cavity. The formation of a mesoderm gives 

 rise to other layers external to this one. Of these the most im- 

 portant is a muscular layer, for by it the intestine is enabled to per- 

 form independent movements. The opening which leads into the 

 enteric tube serves as a mouth for the ingestion of nutrient matters, 

 as well as for an opening for the rejection of the undigested remains of 

 the food (Ccelenterata, many Vermes). The appearance of an anal 

 orifice produces a further separation of functions, and converts the 

 blindly- ending gut or enteron into a tube open at both ends, the 

 separate portions of which take on various functions, and so undergo 

 different adaptations. The first portion, which is connected with the 

 mouth, forms an oesophagus, which serves for the introduction 

 of food ; then follows the true digestive cavity, which is generally 

 widened, or provided with cascal sacs, and is generally called the 

 stomach, though this name is not always applied to equivalent 

 parts. The terminal part of the whole system serves for further 

 alteration of the food, as well as for the excretion of the remnants 



