218 



PLATYHELMIXTHES. 



metamorphosis, the larvae, known as Miiller's larva, possessing finger- 

 shaped ciliated lobes (Fig. 174). 



Asexual reproduction by transverse fission takes place in the 

 Microstomidae (Fig. 175). The posterior third of the body becomes 

 separated from the rest by a septum, and develops its own mouth 

 and organs. This process may continue both in the offspring and 

 parent before separation 

 takes place, so that a com- 

 plex chain of individuals 

 arises (Fig. 175). 



Planaria alpina is also 

 known to undergo fission. 

 The power of regenerating 

 lost parts is great. 



Order 1. ACOELA.* 



FIG. 174. Larva of Eurylepta 

 auricnlata (after Hallez). 



Small marine Turbellaria 

 without digestive cavity, but 

 with a spongy digestive paren- 

 chyma not differentiated into 



endoderm and mesoderm. Dejinite excretory organs 

 have not been Observed. A single otocyst is alirai/* 

 present. Mouth ventral, leading into a short pharynx. 



The central nervous system, generative organs, and 

 digestive organs, are not sharply separated from the 

 parenchyma, but appear rather to be a part of it. 

 Generative ducts are not specially developed, though 

 in the male organs there appear to be fairly -well 

 specialized tracts along which the sperm passes. The 

 genital opening is double or single ; a penis is always, 

 and a bursa seminalis sometimes present. There are 

 no yolk-glands, and the excretory system has not been 

 made out. There is always a pharynx, which ends 

 in the central mass. At the front end of the body 

 there is an organ of a glandular or sensory nature, which was 

 formerly mistaken for the mouth. It is the "frontal organ." 



The Acoela are marine. Most are carnivorous, but it is said that 

 some species of Convoluta can live like a green plant by means of the 

 green cells (1 symbiotic algae}, which live symbiotically in their tissues. 



* Y. Delage, "Etudes histologiques sur les Planaires Rhabdocoeles Acoeles," 

 Arch. zool. exp. et gin. (2) T. iv. 1886. 



FIG. 175. Micro- 

 stomuin lineare 

 (after Graff). A 

 chain produced 

 by fission ; 0, 0' 

 mouth open- 

 ings. 



