3 66 



HISTORY OF THE HUMAN BODY 



bules (nephridia) and germ glands (gonads). (Fig. 105, 

 B.) If the animal is unsegmented, i. e., consists of a single 

 segment, there is a single pair of each ; if it is multisegmented, 

 there is a pair of each for each segment. 



Each nephridium consists of a free tubule whose function 

 is to extract from the protoccele certain waste products in 

 liquid form, a function which it performs in part by a ciliated 

 funnel-shaped opening, the nephrostome, and in part by the 

 physiological action of the cells of which its walls are com- 

 posed. In its simplest form it is straight or slightly curved, 

 but it is more usually coiled in order to increase its length, 



B 



FIG. 105. Diagrams to illustrate the prevertebrate history of the nephri- 

 dia, the gonads and the ccelom. 



(A) Stage of gastrula, with two germ layers, a gastrocoele (g), and a primary 

 body cavity (/>). (B) Here a third layer has appeared in the form of paired 

 gonadic sacs (m), and paired nephridial tubules (f) ; with external openings at 

 o and x respectively. The nephridia are furnished with an inner opening, the 

 nephrostom (n). (C) In this the gonadic sacs (m) have expanded and form the 

 definite ccelom, limiting the primary body cavity to a series of small spaces in all 

 parts of the body. The nephridia open internally into these sacs, and their outer 

 ends open into a longitudinal duct (#). 



and hence its functional efficiency within the prescribed limits. 

 Nephridia of this type are frequent among invertebrates. 



The other sort of organ, the gonad, has the form of a 

 simple epithelial sac, with a narrow duct. Its walls are con- 

 stantly proliferating and furnish cells which project into the 

 interior and finally become free, passing out through the 

 duct. These are the germ-cells, and may be either ova or 

 spermatozoa, the product respectively of female and male 

 parent individuals. Gonads of this character are frequently 

 found among invertebrates, often in quite typical form. 



