1 66 PHYLUM PORIFERA — SPONGES 



Sir E. Ray Lankester in the term '' Parazoa." He also 

 speaks of them as a sterile stock. 



Their origin is wrapped in obscurity ; it may be that 

 they are the non-progressive descendants of primitive 

 gastrula-like ancestors with a sluggish constitution. The 

 presence of choanocytes suggests a relationship with certain 

 of the flagellate Protozoa (Choanoflagellata), and Protero- 

 spongia (Fig. 71) may possibly be regarded as a connecting 

 link. 



Incertte 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 ciUated outer layer, enclosing a single multi- 

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

 without fertilisation, into dimorphic embryos (see Fig. 81, A). 



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

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

 sists 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. 81, B). 



3. Professor F. E. vSchulze discovered a small marine organism — 

 Trichoplax adhcsrens — in the form of a thin, three-laiyered, externally 

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

 Treptoplax adhcsrens. But Trichoplax is now said to be the planula 

 of the Hydromedusan Eleutheria. 



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

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

 one layer of cells (Fig. 82). 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 animal reproduces by trans- 

 verse 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. 



