THE TURBELLARIA 



It appears that when discharged, the first two structures swell 

 up, or contribute to form the slimy material with which the animal 

 invests itself on irritation. It is still uncertain what is the chief 

 function of the rhabdites ; whereas Max Schultze regarded them 

 as serving to increase the sensitiveness of the skin, others believe 

 they give a firmness to the body. It should be mentioned that a 

 considerable number of Rhabdocoela have no rhabdites ; and that 

 these are frequently commensal or parasitic, viz. species of Proxenetes r 

 Graffilla, Fecampia, Acmostoma ; and amongst non-parasitic forms, 

 Cylindrostoma, Plagiostoma, Prorhynchus. 



Below the epidermis is a distinct basement membrane, into- 

 which the muscles are attached (IV. d in all figs.). In the Rhab- 

 docoelida the musculature consists of an outer layer of circular 

 fibres and a deeper layer of longitudinal ones, between which in 

 larger forms is a layer of diagonal fibres. In addition, dorso- 

 ventral muscle cells traverse the parenchyma in a more or less 

 definite way ; they are only feebly developed in the Acoela. The 

 muscle cells are all smooth, and those running dorso-ventrally are 

 branched at each end. 



The characteristic connective tissue of the Platyhelmia occu- 

 pying the space between the dermal muscles and the viscera is 

 termed "parenchyma" (or mesenchyma, Bohmig). This meso- 

 blastic tissue gradually fills the blastocoele of the embryo. In 

 the adult Rhabdocoelida it appears to consist of branched cells,, 

 which may be vacuolated, and so give rise to intracellular spaces 

 (Fig. IV. 4) ; the processes of the cells unite with their neighbours 

 and enclose intercellular spaces or lacunae, which may communicate 

 with one another and so form a sort of rudimentary lymphatic or 

 vascular system ; this seems to be a fairer comparison than to- 

 regard these spaces as coelomic ; the true nature of this tissue is, 

 however, by no means agreed upon by the various authorities ; and r 

 indeed, it appears to differ in the different orders of Turbellaria. 

 In this connection reference may be made to the peculiar con- 

 nective tissue of the Mollusca, in the interpretation of which 

 precisely similar antagonistic views have been held by various 

 authorities. In the lacunae is a fluid which is frequently coloured ; 

 and in Syndesniis (Cuenot) and Derostoma it is said to contain haemo- 

 globin. The parenchymal cells themselves are frequently loaded 

 with pigment granules, giving colour to* the animal. In some 

 Alloiocoela this parenchyma is almost entirely confined to the 

 marginal region of the body, and may even form a definite layer 

 immediately within the longitudinal muscles (59). In many of 

 the Acoela this peripheral parenchyma differs from that occupying 

 the greater part of the body (central parenchyma) which is of 

 much looser character, but it again gradually becomes more com- 

 pact as the axis of the body is approached (see Bohmig, 6); and in 



