124 MESOZOA. 



1. Dicyemidse (type Dicyema) occur as parasites in Cephalopods ; 

 the body consists of a ciliated outer layer, enclosing a single multi- 

 nucleate inner cell, within which egg-like germs develop, apparently 

 without fertilisation, into dimorphic embryos (see Fig. 53, A). 



2. Orthonectidae (type Rhopalura) occur as parasites in Turbellarians, 

 Brittle-stars, and Nemerteans ; the body is slightly ringed, and consists 

 of a ciliated outer layer, a subjacent sheath of contractile fibres, and an 

 internal mass of cells, among which ova and spermatozoa appear. The 

 sexes are separate and dimorphic (see Fig. 53, B). 



3. Professor F. E. Schulze has discovered a small marine organism 

 — Trichoplax adherens — in the form of a thin, ^res-layered, externally 

 ciliated plate ; and Monticelli records a similar form under the title 

 Treptoplax adhcerens. 



4. Professor J. Frenzel has discovered in brine solutions a minute 

 Turbellarian-like organism — Sattnella salve — whose body consists of 

 one layer of cells (Fig. 54). There is an anterior mouth, a ciliated 

 food canal, and a posterior anus. The ventral surface is finely ciliated, 

 the rest of the cells bears short bristles. The animal reproduces by 

 transverse fission, but conjugation and encystation also occur. 



