22 BULLETIN OF THE 



thicker on the ventral side (Fig. 10) than on the dorsal (Fig. 1). In 

 cross sections the longitudinal muscles appear separated into bundles, 

 between which the ends of the dorso-ventral fibres are seen passing to 

 - the basement membrane, into which they are inserted. I have not been 

 able to find ja nucleus in or on either the circular or longitudinal mus- 

 cle fibres. The nucleus of the dorso-ventral fibres is eccentric, as in the 

 muscles of Planarta torva, figured by Ratzel ('69, p. 275, Taf. XXIII. 

 Fig. 26). In cross sections both circular and longitudinal fibres have 

 an irregular outline and show a differentiation into an outer highly re- 

 fractive contractile portion and an inner feebly refractive axis (Plate I.). 

 Branching ends were observed oidy in the sagittal fibres. 



A reticulate mesenchyma constitutes the greater portion of the sub- 

 stance of the body, occupying all the spaces between the organs. The 

 spaces left by the irregular network formed by the branching cells 

 (Plate II. Fig. 18) are connected with one another, and are to be con- 

 sidered as a kind of pseudocode ; they are filled with a granular peri- 

 visceral fluid. The sagittal muscle fibres in some places appear to be 

 directly continuous with branches of the mesenchyma cells (Plate II. 

 Fig. 18, mu. sag.), so that by contraction of the muscles the sizes of the 

 spaces would be altered, and thereby the perivisceral fluid would be set 

 in motion, thus establishing an irregular circulation in the pseudocode. 

 Lang ('84, p. 83) maintains that in the case of Polyclads these spaces 

 are intracellular in their origin, and that the so called perivisceral fluid 

 is the result of a liquefaction of the plasma of the connective-tissue cells, 

 which thus become vesicular, and finally, by the breaking through of 

 their thin walls, form a network. But if the pseudocoslar spaces were 

 intracellular in their origin, as claimed by Lang, it would be more diffi- 

 cult to understand the intimate relation between the muscles and the 

 processes of the reticulated parenchyma cells ; it would not, however, be 

 in any way an exceptional condition for muscle fibres to be attached to 

 the prolongations of stellate connective-tissue cells, more especially when 

 we consider that the muscle fibres and mesenchyma cells have a common 

 origin. As is well known, the Hertwigs have produced evidence to 

 show that the mesenchymatous muscles of the Pseudocoelous animals 

 are " besonders differenzirte Zellen der Bindesubstanz " ('81, p. 98). 

 Moreover, the mode of origin maintained by Lang is not founded, as far 

 as I understand it, on evideuces from embryonic conditions. Graff ('82, 

 p. 72) was unable, from the evidence found in Rhabdocceles, to estab- 

 lish " a distinction between muscle fibres and connective-tissue fibres " ; 

 and Hamann ('85, p. 96) has shown that in Echinoderras the connective- 



