218 



PLATYHELMINTHES. 



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

 shaped ciliated lobes (Fig. 174). 



Asexual reproduction, by transverse fission takes place in the 

 Microstonuilae (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 corn- 

 plex chain of individuals 



arises (Fig. 175). 



TO 7 i 

 Planaria alpina is also 



known to undergo fission. 

 The power of regenerating 

 lost parts is great. 



Order 1. ACOELA.* 



FIG. 174. Larva of Eurylepta 

 aunculata (after Hallez). 



marine Turbellaria 

 irithout digestive cavity, but 

 ii-ith a spongy <1i< festive paren- 

 chyma not differentiated into 



endoderm ami mesoflerm. Definite excretory organs 

 have not been observed. A single otocyst is always 

 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 (I symbiotic algae), which live symbiotically in their tissues. 



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

 Arch. zool. exp. ct gtn. (2) T. iv. 1886. 



FIG. \15.-Micro- 



stomum lineare 

 (after Graff). A 

 chain produced 

 by fission ;0, 0' 

 mouth open- 

 ings. 



