TURBELLARIA. 313 



shelled or summer eggs invariably precedes that of the hard-shelled 

 or winter eggs, and the summer eggs are normally self-fertilized. 



In rare cases the hermaphrodite generative organs present a 

 segmentation recalling that of the Cestoda (Alaurina composita). 



The freshwater Turbellaria, as well as many of the marine forms, 

 undergo a simple direct development, and in the young state are often 

 difficult to distinguish from Infusoria. Other marine Dendrocoela 

 undergo a metamorphosis, the larvae being characterised by the 

 possession of finger-shaped ciliated lobes (fig. 251). 



(1) Sub-order : Rhabdoccela. The body is round and more or less 

 flat. The intestine is cylindrical, and there is usually a protrusible 

 pharynx. They are usually hermaphrodite. 



The Rhabdoccelous Turbellarians are the smallest and most simply 

 organised forms. The intestine is cylindrical and elongated, and is 

 sometimes provided with lateral diverticula. The position of the 

 mouth varies exceedingly, and has been employed as a principal 

 characteristic for distinguishing the various families. Sometimes 

 salivary glands are present, opening into the pharynx. According 

 to Ulianin's discovery, which has been several times confirmed, the 

 alimentary canal may be wanting in many forms, and be replaced 

 by a central cavity, filled with a substance containing numerous 

 vacuoles and rich in oil globules (Convoluta, Schizoprora, Nadina). 

 In those Rhabdocoela which possess an alimentary canal, interstices 

 and spaces in the connective tissue parenchyma are often present : 

 these must be related to a body cavity. In some cases (in Prostomum) 

 the body cavity may be recognised as a continuous space filled with 

 fluid and surrounding the alimentary canal. The Rluibdoccda live 

 on the juices of small worms and of the larvae of Entomostraca and 

 Insecta, which they envelop with a cutaneous secretion containing 

 small rods, and afterwards suck. They are mostly inhabitants of 

 fresh water, and only a few of them are to be met with in the sea 

 or upon the land (Geocentrophora sphyrocephcda). 



Fam. Opisthomidae. The mouth is placed at the posterior end of the body 

 and leads into a . tubular pharynx, which can be protruded like a proboscis. 

 Monocelis agilu M. Sch., Opisthomum pallid-urn O. S. 



Fam. Derostomidae. Mouth placed slightly behind the anterior margin ; 

 pharynx barrel-shaped. Derostomum Scltmidtiamtm M. Sch., Vortex viridis, 

 M. Sch., Catcnnla lemncc Dnir. 



Fam. Mesostomidae. Mouth placed nearly in the middle of the body, 

 pharynx ringlike, cylindrical or resembling a sucker. Jfcsostomum Ehreribcryii 

 Oerst., with two eyes. 



Fam. Convolutidae. (Acoela). Without alimentary canal. The ovaries and 



