CNIDARIA AS THE ONLY COELENTERATA 71 



this led to the typical circlet of tentacles which serves as the 

 main appliance with which they catch their food); (3) the spe- 

 cialization of the monocellular skin glands with their solid or 

 outflo\\ing secretion (here rhabdites and sagittocysts; cnidae 

 in cnidaria); (4) the tendency of the aproctous intestine to 

 develop diverticula (excrescences) and folds and thus to in- 

 crease the internal surface (this is so characteristic and manifold 

 in Turbellaria that it has been used as a criterion on which, 

 their classification has been founded: Acoela, Rhabdocoela, 

 Tricladida, Polycladida; in Anthozoa it is best developed in the 

 polyps of Cnidaria in the form of sarcosepta, or of longitudinal 

 folds, yet here we can observe a later gradual regression with 

 exception of the diverticula which grow into the tentacles; an 

 even richer development can be observed in medusae); (5) the 

 inclination to form a cuticle; in Turbellaria this can be observed 

 occasionally only and it is usually strictly localized; it is only 

 in Temnocephala that it covers almost the w^hole surface of the 

 body; in Anthozoa its occurrence is less extensive; in Hydrozoa 

 w^hich have adopted a completely sessile way of life, it becomes 

 a generally accepted property (here it is known as the periderm 

 formation); (6) the same is true also for the impregnation of 

 the cuticle (periderm) with calcium salts; this can also take 

 place, even if to a lesser degree, in the mesenchymal cells; in 

 Turbellaria this happens to a very Umited degree, a fact which 

 can be explained as being due to their free mobility; (7) the 

 inclination to transversal division; in Turbellaria this has been 

 inherited from their supposed infusorial ancestors; even if this 

 is— as well as several other properties that have been mentio- 

 ned—not a quality that generally appears, it is nevertheless a 

 common property, i.e. it depends on the same gene or on the 

 same complex of genes; in Scyphozoa it has led to the develop- 

 ment of a form of medusae, and in Anthozoa to the first step in 

 the same direction fF///?g/<2J . Many other facts could be mention- 

 ed in this connection, yet the above points should suffice. 

 Whatever is generally valid for the relationship Turbellaria- 

 Cnidaria, the same can also be proved valid for the evolutionary 

 6* 



