240 MANUAL OF ZOOLOGY. 



often evertible ; and the intestine may be either straight or 

 branched, but always terminates csecally behind, and is never 

 provided with an anal aperture. The " water-vascular system " 

 communicates with the exterior by two or more contractile 

 apertures. The nervous system consists of two ganglia, situ- 

 ated in front of the mouth, united by a commissure, and 

 giving off filaments in various directions. Pigment-spots, or 

 rudimentary eyes, from two to sixteen in number, are often 

 present, and are always placed in the prse-oral region of the 

 body. The male and female organs are united in the same 

 individual, and the process of reproduction may be either 

 sexual, by means of true ova, or non-sexual, by internal gem- 

 mation or transverse fission. 



The Planarians have been divided into two sections, as 

 follows : 



Section A. RHABDOCCELA. Intestine straight, not branched j 

 body elongated, rounded, or oval. 



Section B. DENDROCCELA. Intestine branched or arbores- 

 cent; body flat and broad. 



SUB-ORDER II. NEMERTIDA. The Nemertida^ or "Ribbon- 

 worms," agree in most essential respects with the Planarida. 

 They are distinguished, however, by their elongated, vermi- 

 form shape, by the presence of a distinct anus, by the posses- 

 sion of a distinct perivisceral cavity, by the absence of an 

 external aperture to the water-vascular system of the adult, 

 and by the fact that the sexes, with one or two exceptions, are 

 distinct. The Nemertida further differ from the other Platyd- 

 mia in possessing a pseudo-haemal system in addition to, and 

 distinct from, the water-vascular system. The external surface 

 of the body is richly ciliated, and is underlaid by a thick 

 glandular cutis, beneath which are well-developed sub-cutane- 

 ous muscles.' The digestive canal is ciliated internally, and 

 consists of a muscular gullet, a sacculated stomach, and an 

 intestine with a distinct anus. The nervous system consists of 

 two large cephalic ganglia, united by a double commissure, 

 and sending lateral cords backwards. The so-called "circu- 

 latory system " is composed of closed contractile vessels, some- 

 times containing a corpusculated fluid. "Along the median 

 line of the dorsum lies a special muscular sheath, containing 

 a complicated proboscis, and a highly organised corpuscular 

 fluid, both the sheath and the proboscis passing between the 

 commissures of the ganglia in front" (M'Intosh). The evert- 

 ible and muscular sheath of the proboscis may be as long as 

 the whole body, and the extremity of the latter may or may 

 not be protected by one or more spines (fig. 118). The sexes 



