INVERTEBEATA, CRYPTOGAMIA, MICROSCOPY, ETC. 89 



body aud of tlie genital organs, and from the divarication of the 

 main trunks of the enteron, of which only a thin branch traverses the 

 space ; at times, of course, these enteric branches dilate, and by their 

 dilatation diminish the extent of the cavity ; it was probably this 

 diminution which gave rise to the view that the parts now under 

 description were contractile, and which led to the nervous system 

 being regarded as circulatory in function. A large number of fairly 

 developed eyes lie around this cerebral space. 



With regard to the other marine Planaria, the author states 

 that in all the sj)ecies of Stylochus aud Planocera which he examined, 

 the only differences which are foimd in them from what he saw in 

 P. Graffii, lay in the different extent of the division of the two 

 halves of the cerebroid mass, in the degree of development of the 

 finely granular portion, and in the relations of the anterior and 

 posterior nerve-trunks. In the Leptoplanida the brain is distinctly 

 bilobed, and the lateral nerve-trunks diverge during their coui'se to 

 be approximated towards one another at the hinder end of the body. 

 In Thysanozoon, Proceros, and the other genera in which there are 

 tentacles or tentacle-folds at the anterior end, the brain lies very 

 much more forward. 



To sum up Lang's anatomical results : the brain always lies, in 

 the marine Demlrocoela, in front of the proboscis and of the mouth ; 

 it is always embedded in the muscles and parenchyma of the body, 

 and . is always more ventral than dorsal in position. All the 

 nerves given off from it gradually pass to the margins of the body. 

 From a histological examination, it is found that the central 

 nervous system is enclosed in a thin structureless capsule to which 

 there is closely applied a layer of muscular fibres ; the constituent 

 portions of the cerebrum ai e ganglion-cells and fibrous bands, and these 

 constituents are always symmetrically arranged around a vertical 

 median plane ; no nerves are seen to be given off at a sagittal plane. 

 The ganglion-cells vary greatly in character, and may be unipolar, 

 bi- or multipolar, the last being the largest, and exceeding in size any 

 of the cells of the body, saving always the ova. The vesicular 

 nucleus is large and has a distinct contour, within which there is a 

 dark and rounded nucleolus ; in the smaller cells the nucleus is of 

 very much the same size as in the larger ones. The ganglion-cells 

 generally form the outer portion of the cerebrum, while the central 

 part is formed of a substance which is made uj) of extremely fine 

 fibres, and is altogether free from nuclei or ganglion-cells. The 

 investigation of sections has convinced the author that the nerves are 

 connected with the ganglion-cells and the fibrous substance of the 

 brain, and tliis point, as well as the fact that the nervous matter fills 

 the cerebral capsule, seems to be sufficient to settle tlie vexed question 

 as to whether the system iinder investigation is nervous or circulatory. 



The nerve-fibres are delicate aud arc stained with difficulty ; 

 nuclei and ganglion-cells, the majority of which arc bipolar, are found 

 embedded in them, and the largest (multipolar) are specially 

 developed at tbe anastomoses. Tlie author was not able to discover* 

 any circulatory or water-vascular system. 



