Miscellaneous. 467 



during the retreat of the animal all these parts act in order to pro- 

 duce the retraction, they do not act all in the same way during its 

 unfolding. Then the portions placed in front of the nervous collar 

 intervene, at least passively, in its protraction. These muscular 

 bands having on one side their points of attachment to the integu- 

 ments, they must, as the latter are carried forward, aid in drawing 

 the collar into this movement if they did not act only as simple 

 ligaments. 



This is not aU; the muscular expansion which surrounds the 

 nervous collar furnishes to the nerves which start from the supra- 

 and suboesophageal ganglia a regular contractile sheath. This is 

 often considerable, and then, if the nerve be examined by the mi- 

 croscope, under a low power, or by the help of a simple lens, it pre- 

 sents the appearance of an opaque, more or less flexuous cord in 

 the middle of this envelope, which constitutes an external neuri- 

 lemma for it, the histological composition of which must detain us a 

 moment. In the first place we find in it a superficial conjunctive 

 layer, formed by voluminous cells, the mean diameter of which is 

 •05 millimetre, and which is, up to a certain point, comparable to 

 the adventitia of the vessels. Eelow this cellular membrane we re- 

 cognize the presence of a muscular layer formed of fine and very 

 elongated fibres arranged longitudinally. It is easy to ascertain 

 the existence of these muscidar elements by macerating, for three or 

 four days, the collar and the nerve which starts from it in a mixture 

 of equal parts of hydrochloric and nitric acids, diluted with ten or 

 twelve parts of water. It is then easy to separate them. 



Immediately round the nerves may be remarked a second con- 

 junctive element, or inner neurilemma, composed of cellular elements, 

 but less voluminous. These cells are about 0"025 millimetre in 

 diameter. 



This double neurilemmatic envelope has not yet been indicated, 

 so far as we know, in the animals under consideration. Leydig has 

 observed it in the Arthropoda and in the Annelida, where the ex- 

 ternal neurilemma is represented by the ventral vessel ; he has re- 

 cognized, particidarly in the earthworm, the presence of muscular 

 elements ; but we have nowhere seen the existence of these elements 

 indicated in the neurilemma of the MoUusca. According to Leydig, it 

 takes that cellular form of conjunctive tissue which is seen elsewhere 

 among the organs. This is only correct with regard to the super- 

 ficial cellular layer ; and what we have said shows how much more 

 complex is the composition of this neurilemma. 



The existence of muscular fibres in the sheath which encloses the 

 nerve has the effect of producing an elongation and shortening of 

 this musculo-nervous cord ; and, indeed, when there is a contraction, 

 the flexuosities described by the nerve in its envelope are more 

 marked the stronger this contraction is ; in the state of relaxation, 

 on the contrary, the nerve follows a rectilineal direction. 



It is plain that this pecidiar musculature of the nerves has a 

 manifest physiological relation with the intimate connexion that 

 we have indicated between the nervous collar and the muscular 



