388 THE EVOLUTION OF THE METAZOA 



it was frequently called a "sack gonad" (Hatschek). There is 

 no gonocoele yet in the most primitive Turbellaria which inight 

 be also true for the phylogenetically oldest Eumetazoa ; these 

 forms do not yet possess a regularly formed gonad but only 

 an irregular aggregation of gametocytes which do not have 

 any gonoducts. The gonocoele can be found highly developed 

 in those animal groups which do not possess a perigastro- 

 coele (our Ameria) as is the case, with the higher 

 Mollusca. This gonocoele can again disappear secondarily, 

 just as is the case w^th the nephrocoele; this takes place above 

 all in those animal types which possess a perigastrocoele, yet 

 it can also be observed in the Ameria. Not infrequently we 

 can find gonocytes which do not form a hollow or a soUd 

 gonad and which become simply attached either to the skin 

 (this can be observed in some Cnidaria), or to the intestinal 

 epithelium (also in some Cnidaria), or even to other coelom 

 cavities, above all to the perigastrocoele. At first sight this 

 fact seems to support the gonocoele theory which maintains 

 that the genuine coelom (the perigastrocoele) evolved from 

 the gonocoele. Such a theory cannot be accepted from our 

 standpoint. 



The beginnings of the third coelom, of the haemocoel in 

 the form of a haemocoelom system can also be found as early 

 as in the Turbellaria. In these it appears occasionally as a 

 system of blood vessels and lymphatic ducts; it can be found 

 well developed first in the Nemertinea and in the Mollusca. 

 It had been inherited by the Annelida as the lowermost Poly- 

 meria from some ancestors which stood closely to the Nemer- 

 tinea. A pulsating centre of the haemocoele, a muscular heart, 

 does not appear earlier than in the Mollusca. 



There are some other special coeloms. Let us mention two 

 such coeloms only ; the rhynchocoele of the Nemertinea, and 

 the cardiocoele (the pericardial cavity) of the Mollusca; these 

 forms thus occur in the Ameria. All these coeloms and 

 systems of coeloms evolved in the middle body layer, usually 

 by way of a continuously progressive evolution from some 



