326 GENERAL CONSIDERATIONS ON THE MOLLUSCA. 



starting points for the Mollusca in which segmentation is altogether 

 wanting. 



The temptation is certainly very great to derive the elongate, 

 worm-like Solenogastres which are provided with a coelom and 

 nephridia (Fig. 47 .4) from the Annelida, but they also show no 

 segmentation. Either segmentation or the distinct remains of it 

 must, however, be found if they are really to be more nearly related 

 to the Annelida. We are thus inclined to regard the elongate form 

 of the Solenogastres rather as a secondary phenomenon and to con- 

 sider as such also the growing out of the posterior part of the larva 

 into the adult body which recalls similar processes in the Annelida 

 (Vol. i., p. 268). It is possible that the development of the Soleno- 

 gastres, when better known, will throw further light upon their origin. 

 In the young Dunderda, seven calcareous plates are said to cover the 

 back, as in Chiton. This stage thus resembles that of Chiton and 

 this perhaps supports the conjecture that we have in the Amphineura 

 a less primitive form than was assumed a priori. The absence of 

 the shell also would no longer have to be regarded as a primitive 

 feature, nor would the slight development or absence of the foot. 

 We still indeed have to take into account the important fact that 

 the shell is wanting in these forms. The covering of the body with 

 spines and the very primitive internal organisation in any case 

 indicate that the Solenogastres are very primitive forms. If we 

 actually have, in them, merely an aberrant branch of the Molluscan 

 stock, this branch in any case diverged very near the root. 



If we are unable to find any direct relations between the Soleno- 

 gastres and the Annelida, such might perhaps be found between 

 them and other divisions of the Vermes, such as the Turbellaria or 

 the Xemertini. The coelom, that very important part of the internal 

 organisation might, as above shown, with some probability be derived 

 from the dilation of the gonads in these forms. The conditions of 

 the coelom in the Mollusca agree in such a striking manner with 

 those in the Annelida that it is difficult to believe that two structures 

 so remarkably alike arose in different ways ; in this case we should 

 have to derive both the Mollusca and the Annelida from Turbellaria 

 or some similar form. This brings us, however, back to our former 

 view as to the racial form of the Mollusca made in connection with 

 the Trochophore larva (p. 321) which, however, was not favourable to 



the derivation of the latter from the Turbellaria. 



Starting from a creature still more simple in organisation than the 



Trochophore, we arrived at the form of the latter and traced the 



