1 62 UNSEGMENTED " WORMS." 



have a well-defined mesodenn, and a mesoderm lined in- 

 ternal cavity distinct from the gut. But the appearance of 

 a well-developed coelome is very gradual. 



It is not at present possible to have much confidence in 

 preferring one arrangement of the many classes of "worms" 

 to another, but it seems useful to separate the segmented 

 Annelids from the unsegmented types. 



Class TURBELLARIA. Planarians, &c. 



Turbellarians are unsegmented " worms" living in fresh, 

 brackish, or salt water. They represent the beginning of 

 definite bilateral symmetry. 



The ectoderm is ciliated, and contains peculiar rod-like 

 bodies (rhabdites), and occasionally stinging cells. A pair of 

 ganglia in the head region give off lateral nerve cords, and 

 there are usually simple sense organs. The food canal has a 

 muscular pharynx, is often branched, and is always blind. 

 In diet the Turbellarians are carnivorous. There are no 

 special respiratory or circulatory organs ; the body cavity is 

 represented at most by small spaces ; the excretory system 

 usually CQnsists of two longitudinal canals whose branches 

 end internally in ciliated (flame) cells. Excepting two 

 genera, the Turbellarians are hermaphrodite, and the repro- 

 ductive organs usually show some division of labour, e.g., in 

 the development of a yolk gland, which seems to have arisen 

 as an over-nourished (hypertrophied) part of the ovary. 



Classification. 



A. Rhabdocoelida. Small fresh water and marine forms. The body 

 tends to be cylindrical. The food canal is very slightly 

 branched or quite straight or absent. 



(1) Acoela. Degenerate forms without intestine, e.. 9 

 Convoluta, which contains green cells, regarded by some 

 as symbiotic AlgDe. 



(2) Rhabdoccela. With straight intestine, e.g., Vortex; 

 Microstoma, a unisexual fresh water genus, with stinging 

 cells, forming temporarily united asexual chains, some- 

 times of sixteen individuals, suggesting the origin of a 

 segmented type ; Graffilla and Attoplodium, parasitic (cf. 

 next class). 



(3) Alloiocoela. With lobed or irregular gut. All marine 

 except one from Swiss lakes \Plagiostoma Lemani}. 



