136 INVERTEBRATE MORPHOLOGY. 



uniformly two simple club-shaped bodies uniting below to 

 form a common seminal vesicle. The female apparatus may 

 consist of a single ovary (ov) combined with a vitellarium or 

 of two such structures, but usually there is a separation of the 

 vitellarium (vi). In the more complicated cases there is but 

 a single small ovary opening almost directly into the genital 

 atrium, which receives also in addition to the introuiitteut 

 organ the ducts of the two vitellaria. Its walls are further- 

 more pouched out into a seminal receptacle and a sac-like 

 cavity which serves as a uterus, while a peculiar muscular 

 sac, lined by a strong cuticle, the bursa copulatrix, serves for 

 the reception of the intromittent organ during copulation. 

 As stated, however, numerous variations from such a condition 

 occur, and it is not possible to describe any one arrange- 

 ment characteristic of all the Rhabdocosls. 



4. Order Tricladea. 



The Triclads constitute a group of forms with very definite 

 structural peculiarities, occurring principally in fresh water 

 (Planaria, Dendrocodurn, Phagocata), though a few forms are 

 terrestrial (Bipalium), and a still smaller number marine 

 (Gunda, Bdelloura*). As a rule they are elongated in form, 

 one of the terrestrial species reaching a length of 2 cm., and 

 are for the most part free-living, though Bdelloura and Syncce- 

 lidium are ectoparasites of the King-crab (Limulits). The 

 mouth is situated in all cases behind the middle of the body 

 and leads into a somewhat capacious pharyugeal pouch (Figs. 

 69, D, and 73, ph) in which lies a muscular cylindrical pharynx 

 capable of protrusion from the mouth-opening. The diges- 

 tive tract at the base of the pharynx divides into three 

 branches, one of which passes forward in the median line, 

 giving off simple or branched diverticula on both sides, while 

 the other two pass backwards on either side of the pharyngeal 

 pouch, giving off diverticula only from the outer side. The 

 intestinal branches, whose number has suggested the name of 

 the order, and their diverticula are imbedded in a compact 

 parenchyma, no well-marked ccelomic spaces being present 

 (Fig. 69, Z>). 



