PLATYHELMINTHES 21 



B. PLATYHELMINTHES, or Flat Worms. 



BY 



J. W. W. STEPHENS, M.D., B.C., D.P.H. 



DEFINITION : Bilaterally symmetrical animals without limbs, the form of which 

 is leaf or tape-like, rarely cylindrical, and whose primary body cavity (segmentation 

 cavity) is absent, the cavity being filled by a mesenchymatous tissue (parenchyma). 



The mouth is either situated at the anterior end of the body, or is shifted more 

 or less backwards on to the flat ventral surface. The alimentary canal consists of a 

 short fore-gut, which is frequently provided with a muscular pharynx, and of a simple 

 forked or branched mid-gut ; there is neither a hind-gut nor an anus ; in one class, 

 the Cestodes, the alimentary canal has entirely disappeared except for muscular 

 remnants in the scolex. 



The INTEGUMENT OF THE BODY consists either of a ciliated epithelium of only 

 one layer (Turbellaria), or of a cuticle and gland-like cells embedded in the 

 parenchyma, or subcuticular layer (Cestodes, Trematodes). The dermo-muscular 

 layer consists of annular, longitudinal, and even diagonal fibres, while the 

 parenchyma is traversed by dorso-ventral fibres. 



The central NERVOUS SYSTEM, which is embedded in the parenchyma of the 

 body, consists of cerebral ganglia, united together in the shape of dumb-bells, and 

 of two or more longitudinal MEDULLARY FASCICLES, often forming transverse 

 anastomoses. Organs of sense usually occur only in the free-living species, more 

 rarely during the free-living stages of a few parasitic species and in a few ecto- 

 parasitic forms. 



[In Platyhelminthes simple eye-spots frequently occur, and in a few an auditory 

 vesicle.] 



BLOOD-VESSELS and definite RESPIRATORY ORGANS are lacking [except in 

 Nemertinea\ ; the EXCRETORY APPARATUS (formerly termed water- vascular system) 

 is typical of the entire class. It commences in the interstices of the parenchyma, 

 with peculiar terminal cells (ciliated funnels), which will be described later (p. 219), 

 the capillary processes of which go on uniting into larger branches^nd finally form 

 two large collecting vessels, which, sometimes separately and sometimes united, 

 open to the exterior through one, two, or numerous pores. 



Nearly all the Platyhelminthes are HERMAPHRODITIC, and in .nearly all there are, 

 in addition to the ovaries producing ova, other glands attached to the female genital 

 apparatus, namely, the vitellaria or yolk glands, which provide a substance termed 

 yolk, which serves as nourishment for the embryo. The fully formed eggs have 

 shells and are " compound, " i.e., composed of the egg or ovarian cell, which is 

 surrounded by numerous yolk cells or their products of disintegration. The two 

 sexual openings usually lie close together, frequently in the fundus of a genital 

 atrium ; they are rarely separated from one another. Shell glands also usually 

 occur (p. 221). 



Reproduction is sexual, often, however, combined with asexual methods of 

 propagation (segmentation, budding). The Platyhelminthes live partly free in fresh 

 or salt water, exceptionally also on land. The greater part, however, live as 

 parasites on or in animals. 



