128 



MESOZOA. 



It will 



parasitic degenerations of Turbellarian-like worms, 

 be enough here merely to notice four types : 



(i.) Dicyemidre (type Dicyema] occur as 

 parasites in Cephalopods ; the body con- 

 sists of a ciliated outer layer, enclosing a 

 single multinucleate inner cell, within 

 which egg-like germs develop, apparently 

 without fertilisation, into dimorphic em- 

 bryos (see Fig. 36, A). 



(2.) Orthonectidne (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. 36 B.). 



(3.) Professor F. E. Schulze has dis- 

 covered a small marine organism Tricho- 

 plax adh<zrens in the form of a thin, 

 three 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 Salinella salve whose 

 body consists of one layer of cells. 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. The larva is uni- 

 cellular. 



These forms are obviously of much 



interest to those who ponder over the possible nature of the earliest 

 multicellular animals. 



Fig. 37. Salinella. 

 (After FRENZEL.) 



1. Longitudinal section- a, 

 anterior ; /, posterior. 



2. Transverse section. 



