108 BULLETIN : MUSEUM OF COMPARATIVE ZOOLOGY. 
the animals studied. There is not much doubt, however, concern- 
ing the origin of the inner spongy layer, the neuroglia. Jourdan (84) 
showed that the enveloping tissue of the central nervous system was 
intimately connected with the cells of the hypodermis. Rohde (87) 
called this tissue “Subcuticularfasergewebe,” and described it as a 
development of the basal processes of the cells of the hypodermis. 
Wawrzik (92) made a comparison of a large number of Polychætes, and 
found that in all those in which the ventral cord was connected with 
the hypodermis the neuroglia was an integral part of the hypodermis 
cells. Haller (89) denies the existence of the condition described by 
Rohde ('87) for Polynoé, since he found that the nerve cord was sur- 
rounded by a membrane which separated the neuroglia from the hypo- 
dermis. However it may be in this case, there certainly cannot be a 
connection between the hypodermis and neuroglia in such forms as 
Hermione, Aphrodite, and Nereis, in which these structures are clearly 
separated. But the condition found in so many other genera indicates 
that the neuroglia is derived from the ectoderm along with the nervous 
elements. 
The neurilemma is apparently found only in those forms in which 
the nerve cord is free from the hypodermis. But even when present it 
may be so thin as to be readily overlooked. Such is sometimes the case 
at the posterior end of Nereis. On the other hand, it becomes very 
thick around the brain of Nereis, sometimes reaching a thickness of 
fully 100%. Friedländer ('88) and Graber (80) call this structure 
cuticular. Haller considers it simply the matted fibres of the neuroglia. 
Racowitza (’96) states that muscle fibres, as well as the neuroglia, con- 
tribute to make up the neurilemma. Where muscle fibres are attached 
to the outer surface of the neurilemma, or neuroglia fibres to its inner 
surface, membrane and fibre shade insensibly into each other, so as to 
suggest their structural identity. But, as has been shown above, as 
well as by other writers, the neurilemma in its reaction to stains is 
‘very different from either muscle or neuroglia. Whatever may be the 
weight of this evidence, it is clear that the neurilemma, the connective 
tissue of the muscles, and the tunica intima of the ventral longitudinal 
blood-vessel have the same structure, and must be derived from the 
same source. That source is most likely the mesoderm. 
3. BRAIN. 
Although the brain of Nereis gives rise to so many nerves, it is 
small and simple when compared with the brain, for example, of the 
