THE NERVOUS SYSTEM. 



295 



The musculature of the head is not always limited to the hitherto 

 discussed groups of fibres. Special, more or less complex, muscular 

 arrangements are brought into requisition for moving 

 the suckers, and also sometimes the hooks, as in 

 Tcenia. The most peculiar modifications are found 

 in Tcenia (Fig. 151) and TetrarhyncJius, in which not 

 only have the suckers a specially strong and complex 

 musculature, but the hooks are seated on a more or 

 less proboscis-like muscular apparatus (four of which 

 are present in TdrarJiynchus), which finds a parallel 

 only in the Acanthocephali. 



Similarly, there are usually special muscles in 

 connection with the penis, to which we shall after- 

 wards return. 



The Nervous System. When we consider the 

 development of the muscles, it seems strange that 

 the nervous system has long been sought for in vain. 

 It is true that some investigators have described a 

 central ganglion in the head of certain species, and 

 especially in Tetrarhynchus, which is distinguished 

 by the large size of that part of the body. I recall 

 especially the observations of J. Miiller and Wagener, F IG i5i._Longi- 

 but they have still left us without any real proof of tudinal section of 



,1 i> ji j_- mi A\ a young Tcenia 



the correctness of their representations. Ihus, the serrata, consisting 

 existence of a nervous system in the Cestodes long mainl y of head and 

 remained a moot-point, and that the more naturally 

 since the reports as to the nature of the ganglionic apparatus contra- 

 dicted one another, and the great majority of observers had to declare 

 themselves unable to find anything which could be with certainty 

 denoted as a nervous system. 



And yet the Cestodes do possess a nervous system, and a well 

 developed one, with a central part lying inside the head, and with two 

 distinct lateral cords which run continuously down the whole chain of 

 joints, and which sometimes, as in the larger species of Tcenia and 

 Tdrarliynchus, break up into several strands running side by side. 

 In Tcenia they lie outside the excretory canals, and are easily 

 detected in transverse sections. That they have hitherto been generally 

 overlooked is largely due to the fact that they are destitute of any 

 independent sheath, and consist of but slightly specialised tissue. 

 They have not, of course, remained wholly unobserved. For not only 

 was the cephalic ganglion described by the older observers in Tetra- 

 rliynckus, partly at least, as really related to the nervous system, 1 



1 See especially Wagener's representation, " Entwickelung der Cestoden," Den 

 naturk. Verhandl. Holland. Maatschappy : Haarlem, pi. iv., 1857. 



