PLANARIANS 



(By Prof. Luduriy -von Graff, University of Graz, Austria.) 



rPHE name Planaria was first applied by O. F. Miiller 

 in his Frodromus Zoologix Danicx (1776) to a 

 group of worms, inhabitants of fresh and salt water, 

 characterized, so far as was then known, by a flattened 

 leaf-like form. Ehrenberg in 1831 changed this name to 

 Tvrbellaria on account of the cilia with which the body is 

 furnished, by means of which the worms create a whirl- 

 pool in the surrounding water. The extent of this 

 group was subsequently more restricted, and at present 

 the name Twlellaria is applied to all those (mainly 

 free-swimming) Platyhelminths whose body is clothed exter- 

 nally with a ciliated epidermis (fig. 9), and which possess 

 a mouth and (with the exception of one division) an 

 alimentary canal, but are without an anus. The Tur- 

 bellarians, excluding the NEMEETINES (q.v.), which until 

 recently were classed with them, form an order of the 

 class Platyhelminthes, and the old name Planaria is now 

 confined to a group of the freshwater representatives of 

 this order. 



Size and External Characters. Many forms of the 

 Turbellarians are so minute as to be hardly visible with 

 the naked eye, while others attain to a length of several 

 inches, and a land Planarian of no less than 9 inches in 

 length has been described by Moseley. The freshwater 

 forms are generally small, the largest representatives 

 of the order being marine or terrestrial. The smaller 

 species are mostly cylindrical, or convex dorsally and 

 flat ventrally ; the anterior extremity is commonly trun- 

 cated and the posterior extremity pointed (fig. 1, a, I). 

 The larger aquatic forms are thinner in proportion to 

 the increasing surface of the body, so that they come to 

 resemble thin leaf-like lameVJaa (d), while the large land 

 I'lanariana instead of increasing in superficies grow in 

 length (e and /), so that they may be best compared to 

 leeches. The larger aquatic forms are frequently provided 

 with tentacles in the shape of paired finger-like processes 

 or ear-like folds of the anterior part of the body (d and 

 g) ; sometimes the tentacles are papillary outgrowths of 

 the dorsal surface ; the land Planarians are often to be 

 distinguished by a crescent-shaped area at the fore end 

 of the body, which is separated off from the rest (/'). 

 In many cases the whole dorsal surface is beset with 

 papilUe (d). The aperture of the month varies greatly 

 in its position ; sometimes it is situated at the anterior 

 extremity, sometimes in the middle of the ventral surface 

 of the body, occasionally quite close to the posterior ex- 

 tremity ; the single common or distinct male and female 

 generative apertures are also situated upon the ventral 

 surface of the body, and the former in rare cases open in 

 common with the mouth ; the genital apertures always lie 

 behind the mouth. Many Turbellarians have a sucker 

 which serves to attach the animal to surrounding objects, 

 or to another individual during copulation. 



Intiyitmcnt. The integument is composed of a single 

 layer of ciliated epithelium ; between the cilia there are 

 often long flagella and stiff tactile hairs and even (in a 

 single instance) chitinous spines ; these latter must be 

 regarded as local thicken- 

 ings of the firm cuticle 

 which covers the epi- 

 dermic cells. The epi- 

 dermic cells are flat or 

 columnar, and are united 

 to each other by smooth 

 opposed margins or by 

 denticulate processes 

 which fit into similar 

 processes in the adjacent 

 cells (fig. 2). Sometimes 

 the epidermic cells are 

 separated by an inter- 

 stitial nucleated tissue. 

 The structure and func- 

 tions of the cells of the 

 epidermis differ, and four 

 varieties are to be found : 

 (a) indifferent ciliated 

 cells ; (I) cells containing 

 certain definite structures 

 (rhabdites, nematocysts) ; 

 (c) gland cells ; and (>!) 

 glutinous cells (Kleb- 

 zellen). The rhabdites 

 are refracting homogene- 

 ous rod-like bodies, of a 

 firm consistency, which 

 are met with in most 



ph. 



, 



, M. Sch.: <-, Mmetus fuscus, 

 Off.; d, Tliysanozoon trochii, Gr., with 

 elevated anterior extremity (after Joh. 

 Schmidt); e, Rhynchodemus terrestrit, O. 

 F. Miiller (after Kennel); /, Bipalium 

 ceres, Mos. (after Hoseley) ; ;/, I'uiy.-i Us 

 cornuta, O. Sch., attached by the iilum-nx 

 (pA)to a dead worm (after Johnson). All 

 the figures of natural size, and viewed 



Turbellaria, and often fm the dorsal surface, 

 fill all the cells of the epidermis ; they are not always found 

 entirely within the cells, but the extremity often projects 

 freely on to the exterior of the body. They are readily 

 extruded from the cells by pressure, and are often found in 

 great abundance in the mucus secreted by the glandular 

 cells (many Turbellarians, like snails, deposit threads of 

 mucus along their track) ; in this case the epidermic cells 

 become perforated like a sieve. In many Turbellarians the 

 rhabdites are chiefly massed in the anterior part of the 

 budy; frequently there are several varieties of rhabdites 

 in one and the same species, some being pointed at 

 both ends, others cylindrical with truncated extremities. 

 These structures are either formed directly in the ordinary 

 epidermis cells as a kind of secreted product of the cell, or 

 in .special formative cells which lie beneath the integument 

 and are connected with the epidermis cells by protoplasmic 

 filaments, by means of which the rhabdites reach the surface 

 of the body. These cells must be regarded as epidermic 



