ANATOMY AND CLASSIFICATION OF THE ARENICOLID^. 471 



T-shaped. It is hollowed out by coelomic spaces accom- 

 panied by blood-capillaries. Externally it is in close contact 

 with the epidermis by means of its ganglionic layer, which is 

 about equal in thickness to the extremely delicate neuropile 

 forming its inner fibrous portion. 



In A. Claparedii and in young specimens of A. marina 

 the intimate structure of the brain is very similar. The 

 anterior lobes are separated from each other ventrally by a 

 coelomic space. At a higher level they become almost con- 

 tinuous, but until the superficial ganglion cells are reached 

 there is a narrow slit continued between the lobes. At their 

 hinder end these lobes are connected by a band of transverse 

 fibres which form a commissure. It is not, however, possible 

 to delimit them exactly, for they are continued into a pair of 

 narrowed longitudinal tracts (continuous dorsally with the 

 anterior lobes by a common covering of ganglion cells). 

 These pass downwards to the margin of the ciliated groove, 

 and then come into contact with a pair of posterior lobes 

 specially related to these sensory organs (PI. 27, fig. 63). 



The anterior lobes are composed of at least two kinds of 

 ganglion cells: (1) large unipolar elements •01 6 mm. in 

 diameter, the nuclei of which have one distinct nucleolus 

 and contain very little chromatin; (2) small cells ("08 mm.) 

 with nuclei rich in chromatin, and with no distinct nucleolus. 

 The distinction, in fact, appears to be exactly the same as that 

 which holds good for the two chief kinds of ganglion cells of 

 the oesophageal connectives and of the ventral cord. Both 

 these kinds of ceils occur in the intermediate region, but in 

 the posterior lobe the second kind are alone present, or at 

 least they preponderate. 



The fibrous part of the brain is composed of an extremely 

 fine neuropile, and, in the anterior lobes, of tracts passing 

 from groups of ganglion cells into the connectives. There is 

 •but little neuroglial tissue, and the neurilemma forms a 

 delicate membrane on the ventral and anterior surface of 

 the brain, and sends processes dorsally which divide in the 

 neuropile. 



