LAND PLANARIANS. 



103 



tials, natural size. The same process of strobilation has 

 been carefully observed by Graff in Microstomum lineare 

 Oersted. In the chain of four individuals (Fig. 67) I indi- 

 cates the division of the first order, and II those of the 

 second order ; at the points in the zooids marked III there 

 are indications of a future third subdivision, and at IV of 

 a fourth ; so that potentially the chain con- 

 sists of sixteen zooids,- and the division is 

 first indicated in the digestive tract which 

 forms subdivisions with septa reaching to 

 the body-walls, while secondary and tertiary 

 mouth-germs appear in the division-sections 

 m", Fig. 67). 



Huxley in his Manual of the Anatomy of 

 Invertebrated Animals states that in some 

 genera of Turbellariaii worms "a difference 

 is observed between the eggs produced in 

 .summer, which have a soft vitelline mem- 

 brane, and those produced later. These so- 

 called winter ova have hard shells. 



The genuine flat-worms are divided into 

 two suborders : Pliabdoccela and Dendroccela. 

 In the former group there is an extensible 

 pharynx, and the digestive tract is not 

 branched. The Rhabdocoela are represented 

 by Catenula, Prostomum, Microstomum, etc. 



The Dendroccela sometimes have two tenta- 

 cle-like continuations of the front end of the 

 body. The digestive canal has one anterior, two 

 posterior large, and many secondary branches, Fig. 67. strobi- 

 and a proboscis. Here belong the Planarians gton"^ MW)O- 



m' 



n 



of fresh and salt water, and the Geoj)lanid(B 

 or land-planarians, represented in the United 

 States by Rhyncodesmus sylvaticus Leidy. The only para- 

 sitic species of the order known are Stimpson's Cryptocce- 

 luin opacum, which infests the sand-cake (Echinaracltnins 

 parma), and Typhlocolax acuminata, which lives on a Holo- 

 thurian (Chirodota) ; while Semper has described Anoplo- 

 dium Schneideri. which lives in the intestines of Stichopus 



