TUEBELLARIA. 



311 



diverticulum of the stomach runs forward above the transverse 

 commissure in a groove between the two cerebral lobes (Leptoplana). 

 In some genera of Planarians, a ring-shaped double commissure has 

 been shown to exist in the brain (Polycelis), and ganglion-like 

 swellings, from which nerves are given off, have been observed on 

 the lateral nerve- trunks (Sphyrocephalus, Polycladus). 



With regard to sense organs, eye spots are tolerably widely distri- 

 buted among the Turbellaria. They either lie in 

 pairs upon the cerebral ganglia or are connected 

 with short nerves given off from the latter. More 

 frequently two larger eyes with refractive struc- 

 tues are developed. Otocysts are but rarely 

 present, e.g., in Monocelis among the Rhabdoccela 

 a single one is present lying upon the cerebral 

 ganglion. The integument is without doubt en- 

 dowed with a highly developed tactile sense ; the 

 large hairs and stiff bristles which project between 

 the cilia may possibly be of importance in this 

 relation. Lateral ciliated pits, which may also be 

 explained as sense organs, are in rare cases present 

 at the anterior end of the body (compare the 

 Nemertinea). 



Mouth and digestive apparatus are never wanting 

 but the former is frequently removed from the 

 ventral surface of the anterior end of the body to 

 the middle or, indeed, even to the posterior region. 

 According to Metschnikoff and Ulianin, a stomach 

 may in some cases be absent (Convoluta, Schizo- 

 prora), and be replaced, as in Infusoria, by a soft 

 internal parenchyma. The mouth leads into a 

 muscular pharynx, which can usually be protruded 

 after the manner of a proboscis. The alimentary 

 canal, of which the internal wall is frequently 

 ciliated, is either forked and then simple or 

 branched (Dendrocoela), or rod-shaped (Ehabdo- 

 ccela). An anus is never present. A peculiar tube capable of being 

 evaginated as a proboscis, and without connection with the pharynx 

 is sometimes also present (Prostomum). 



The excretory (water-vascular) system consists of two transparent 

 lateral trunks and innumerable side branches, which begin with 

 closed ciliated funnels, and are furnished with vibratile cilia, which 



FlG. 24S. M'crogfo- 

 mum lineare, after 

 Graff. A chain 

 produced by fis- 

 sion ; O,-0', mouth 

 openings. 



