ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 387 



Minot and Hallez are quite right in believing that the interspaces 

 between all the organs are completely filled up with this substance ; 

 the appearance of spaces, such as was seen by Keferstein in his sec- 

 tions, is obviously due to imj)erfect conservation. 



In the chapter on the digestive apparatus we have an account 

 of the external mouth of the pharyngeal pouch, which is separated 

 from the intestine by a " diaphragm," and of the pharynx, the origin 

 of which is explained by some diagrammatic figures ; the gastro- 

 vascular apparatus, as the rest of the digestive tract is called, is 

 described in considerable detail ; the functions of this region would 

 appear to be manifold, for not only has it a digestive function, but 

 Lang agrees with Graff in ascribing to it a respiratory one also. 



The investigation of the excretory or water-vascular system 

 was attended with considerable difficulties ; the author was able to 

 convince himself that no connections obtained between the central 

 cavity of the excretory cell and the lacunae in the parenchyma of the 

 body. On the whole^ this system in the Polyclads agrees with the 

 typical arrangement of other Turbellaria and of the Platyhelminthes ; 

 its highly ramified condition is to be explained as due to the absence 

 of a special body-cavity and of a blood-vascular system. " The 

 excretory system is compelled to seek out the excretory products in 

 every part of the body." 



The historical review of what has been taught as to the nervous 

 system, similar to that which precedes the discussion of all the organs, 

 is of considerable length. The cerebrum is a transversely oval mass of 

 some size, and is indistinctly divided behind into two lobes ; it gives 

 off a number of nerves which are large in proportion to its size, so that 

 the origin of these is somewhat difficult to follow out. At a short 

 distance from the cerebrum the ten strongest nerves are all connected 

 by a commissure ; the six anterior nerve-trunks soon branch and 

 anastomose ; the two longitudinal trunks give off a number of trunks 

 which supply all the hinder parts of the body, and, like the rest, are 

 connected by a number of anastomoses. The ganglionic cells of the 

 central organ may be multi-, bi-, or unipolar, and vary considerably 

 in size ; the largest are the multipolar, and these are larger than any 

 other cells of the body, with the exception of the ova ; the nucleus 

 is in all cases large and vesicular. The central part of the brain is 

 occupied by a finely fibrous substance, in which no nuclei or ganglia 

 can be made out, and the constituent fibres anastomose with one 

 another. The nerves themselves are composed of extremely delicate 

 fibres, which are only feebly stained by reagents. 



The sensory organs may be tactile, optic, or auditory ; all the 

 Polycladidea do not possess tentacles, for they are absent in the Lepto- 

 planida and the Cestoplanida ; some of the former have, however, 

 rudimentary tentacles. The tentacles may be either dorsal in position 

 and confined to the anterior part of the body (" nape-tentacles "), or 

 they may be marginal. In the Planocerida the former are always 

 movable, and in all cases they may be regarded as parts which have 

 been inherited. The marginal tentacles, on the other hand, are 

 structures which clearly have arisen since the Polyclad-stock was 



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