PHYSIOLOGY OF THE INVERTEBRATA. 427 



As a rule, the male organ consists of a single csecal tube 

 opening on the ventral side into the cloaca or the posterior 

 end of the intestine. The spermatozoa,* which are amoeboid 

 in shape, are developed from the blind end of the caecal tube, 

 whose remaining portion has the function of a vas deferens. 

 One or two, sometimes long, chitinous spicula are developed 

 in the cloacal region of the male. These spicula are used by 

 the male during copulation — the object being to distend the 

 vulva of the female, in_order to allow the seminal fluid to 

 pass freely into the vagina and uterus. The spermatozoa 

 undergo further changes in the female organs of reproduction, 

 but ultimately fuse with the ova. 



The female organ consists of a single or double tube, which 

 is blind at one end. The blind end of this tube contains 

 internally a protoplasmic substance or rhachis, from which 

 the ova are developed ; this portion of the tube is, therefore, 

 physiologically the ovary of other forms. The tube then 

 becomes differentiated into an oviduct, and later into a 

 uterus. The ova are free in the oviduct, and they are im- 

 pregnated in the uterus, where they become surrounded by a 

 hard shell. The uterus then passes into the vagina, which 

 opens on the ventral surface, usually near the centre of the 

 body. 



The vitellus of the fecundated ovum becomes segmented, 

 and gives rise to a single row of cells, which ultimately 

 become indented on one side — i.e., the ovum forms a kind of in- 

 vaginated gastrula.f The body wall and the alimentary canal 

 are developed from two layers of cells, which are produced by 

 the invagination of the above-mentioned single layer. At 

 this point the embryo rapidly assumes the adult form ; and 

 is found rolled up within the shell. After hatching, the 

 young Nematode casts its cuticle, which is shed a second 

 time when it acquires its sexual organs — i.e., there is a period 



* In the Nematoidea, the spermatozoa retain the character of cells. 

 ' t The ova of Aacaris dentata and Oxyuris amUgna are unsegmented after 

 fecundation. 



