THE TURBELLARIA 23 



(more rarely a single one) and a pair of vitellaria, usually compact, 

 but the latter may branch and anastomose to form an apparently 

 single reticulated organ, but the two ducts indicate its double 

 nature. The accessory female organs are no less varied (Fig. 

 VIII.). A spermatheca, either as a swelling on the common duct, 

 or finally as an outgrowth of the atrium, may be present. A 

 bursa copulatrix (or vagina) may exist or not ; but there is 

 usually a sac in the Rhabdocoela in which the cocoon is retained ; 

 this function is originally performed by the atrium, but in most 

 cases a diverticulum of this chamber, provided with glandular 

 walls, receives the name "uterus." 



Reproduction. Our knowledge of the development of the Rhab- 

 docoelida is very scanty ; in most genera each egg cell becomes 

 enclosed together with numerous yolk cells within a hard capsule, 

 which is secreted by special gland cells. Sometimes the egg is 

 only set free by the death of the parent. This capsule, which has 

 characteristic shapes, is attached to water plants. Segmentation 

 results in the formation of micromeres and macromeres ; gastrula- 

 tion is effected by epibole, and when the embryo has attained a 

 stage with a distinct gut and pharynx it devours the surrounding 

 yolk cells. 1 It is important to note that the animal is at first 

 nearly spherical, and the mouth central ; but by differentiation in 

 growth the mouth becomes carried forwards or backwards as the 

 case may be. 



In some forms (Mesostoma) the winter and summer eggs differ ; 

 the summer ones, having a thin shell, undergo development 

 in the parent's body, as is the case in the Crustacean Daphnia. 

 But perhaps the most interesting fact is that amongst the 

 Microstomidae a mode of asexual reproduction takes place during 

 summer, and the genital organs only mature in the autumn. 

 The fact that Turbellaria can reproduce in this way seems to 

 have been observed first by Dreparnaud (1803); but in 1822 

 J. E. Johnson made further observation on the matter, and M. 

 Faraday (16) carried out some interesting systematic experiments 

 on this subject; both these authors were greatly in advance 

 of their contemporaries in this matter. This asexual process 

 resembles, in the main, that which occurs amongst the Naids 

 (Oligochaeta) and Syllids (Polychaeta), in that, after attaining 

 a certain size, the animal becomes partly constricted ; an active 

 production of cells takes place at the anterior end of this 

 young zooid, by which a new brain and a new pharynx are 

 formed. After this process has gone a certain length, a new 

 constriction and new budding take place near the end of each of 

 the two zooids ; in this way is formed a chain of four, eight, or 



1 In the acoelous Polychoerus there is no trace of an enteron (see Gardiner, Journ. 

 Morph. xi. 1895, p. 155).. 



