28 THOS. H. MONTGOMERY, 



tissue with its rather more mobile (certainly less dense) intercellular 

 substance, should be regarded as the more primitive (ontogenetically) 

 tissue of the two. 



3) Parenchym tissue. This tissue is absent in the head and 

 oesophageal regions, posterior to which it produces a layer of cells 

 around the proboscis sheath (outside of its circular musculature, 

 Fig. 32 Par). More anteriorly, but a single layer of parenchym cells 

 covers the ventral surface of the sheath, but around its posterior 

 portion the cells are two or three deep, and also larger, and occur 

 on its whole circumference though most abundantly laterally and ven- 

 trally. The cells on the ventral periphery of the proboscis sheath, 

 are also in contact with the dorsal surface of the dorsal blood vessel ; 

 but, as in the other Metanemertean species examined, no parenchym 

 cells are found around the lateral vessels. The cells are comparatively 

 large, spherical or polygonal (accordingly as they are isolated, or 

 massed together), the easily discernible cell wall surrounds the un- 

 staining, structureless fluid, with which the greater part of the cell is 

 filled. The nucleus is surrounded by a small mass of finely alveolar 

 cytoplasm, and placed against the cell wall. 



4) Intracapsular connective tissue of the central 

 nervous system. Owing to the minuteness of the elements of 

 the nervous system in the Metanemerteans, and to the great difficulty 

 (apparently even impossibility !) of obtaining a satisfactory method for 

 differentiating them by stains, it is difficult to determine the structure 

 of the ganglion cells, and of the connective tissue elements; and as 

 Bürger ('90) has noted, the outer is more closely applied to the 

 inner neurilemma, and the layer of the ganglion cells comparatively 

 much thinner than in the Anopla. The intracapsular connective cells 

 are very small, membraneless, and apparently multipolar, though their 

 fibres are of such exceeding fineness, that I cannot state with cer- 

 tainty that they are multipolar (Fig. 35). The nucleus is centrally 

 situated, with a relatively small amount of chromatin, the latter mainly 

 on the periphery; it is usually oval or elongate in shape, but fre- 

 quently more or less irregular in outline. These nuclei may be 

 distinguished from those of the ganglion cells, by their more irregular 

 and elongate form; and from those of the neurilemmatic sheaths, by 

 staining less intensely with haeraatoxylin. As in Cerebratulus., three 

 modifications of these cells are found: 



a) Pigmented cells in the brain lobes, between the outer and 

 inner neurilemma (Fig. 35). This greenish-yellow pigment occurs 



