214 DR. A. A. W. HUBRECHT. 



which are sometimes developed in the cuticle side by side with the 

 calcareous spicules, and which may in certain species {Chiton 

 pallasii) attain a rather considerable size. 



The calcareous spicules, both in the Solenogastres and the 

 Chitones, are of very different sizes and shapes (3, 7, 13, 18, 22). 

 In Proneomenia they are of the most uniform shape throughout 

 the whole of the integument (7) ; in Chiton they present the 

 greatest degree of diversity (16, 18). 



For certain genera it has been proved that the spicules remain 

 attached to the cellular matrix, even when situated high up in 

 the cuticle close to the outer surface, by a string of cellular 

 tissue (7, 18). A cellular capsule encloses their base in Pro- 

 neomenia ; in this they find their origin when it still forms part 

 of the subjacent cellular matrix ; they appear to be lifted and the 

 string to grow in length, together with the increase in thickness 

 of the cuticle, which pushes them outwards passively. Similar 

 radial outgrowths of the cellular matrix, which, however, appear 

 to be in no direct connection with the spicules, are figured by Kowa- 

 levsky for Neomenia gorgonophila (13). Very numerous radial 

 hollow tubes in the shells of Chiton, first noticed and figured by 

 Marshall (15), are, moreover, filled during life by strings of 

 tissue, which are direct radial prolongations of the cellular 

 matrix,^ and have great analogy to the funicles above mentioned 



The genus Chitonellus is characterised by its inconspicuous 

 dorsal shells, calcareous spicules being distributed in a very 

 regular way in the rest of the skin. This genus was long looked 

 upon as representing a reduced stage in comparison with Chiton ; 

 different details of its organisation (branchise, foot, &c.) show 

 the inconsistency of this proposition, and of all Chitones it must 

 certainly be looked upon as the more primitive and the more 

 closely related to the Solenogastres (8). A study of the exact 

 structure and growth of its shells is a great desideratum, 

 especially if very young stages of Chitonellus, which at present 

 are so exceedingly scarce in zoological collections, are available. 



Two words may here be added concerning the foot, which 

 makes its appearance in the Solenogastres as a median ventral 

 folding of the integument, not covered by chitinous cuticle and 

 spicules, but ciliated (4, 7, 13), and extending in Neomenia and 

 Proneomenia from close behind the mouth down to the anus. 

 In Chatoderma (4) it is only developed in the posterior half of 



I I have been able to examine sections through the decalcified shells 

 of Chiton made by Doct. Phil. J. F. van Bemmelen, and have satisfied myself 

 that they show the peculiarities alluded to above very clearly. This 

 gentleman being at present occupied in investigating more fully the 

 integument of the Chitones, I here refrain from further details, for which 

 I refer to his paper, which will be shortly forthcoming. 



