132 MESOZOA. 



INCERT/F; SEDIS. MESOZOA. 



The title Mesozoa was applied by Van Beneden to some simple 

 organisms which appear to occupy a very humble position in the 

 Metazoan series. He regarded them as intermediate between Protozoa 

 and Metazoa ; but others have remarked on their resemblance to 

 Platyhelminthes, and especially to the sporocysts of certain Flukes. 

 They may perhaps be regarded as precociously reproductive sporocysts. 

 It will be enough here merely to notice four types : 



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. 60, A). 



2. Orthonectidrc (type Rhopahtra] occur as parasites in Turbellaiians, 

 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. 60, B). 



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

 - Trichoplax adJucren s in the form of a thin, //w^-layered, externally 



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

 Treptoplax adhterens. 



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

 Turbellarian-like organism Salinella salve whose body consists of 

 one layer of cells (Fig. 61). There is an anterior mouth, a ciliated 

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

 the other cells bear short bristles. The animalreproduces by transverse 

 fission, but conjugation and encystation also occur. 



It must be confessed that some corroborative evidence in regard 

 to this peculiarly simple animal is much to be desired. 



