320 GENERAL CONSIDERATIONS ON THE MOLLUSCA. 



through actual relationship to one another of all those groups which 

 have the Trot-hophore as their larval form. 



If we accept this latter view, there can be no doubt as to the 

 significance to be attributed to the larval form. Its reappearance in 

 the development of phyla so different as the Annelida, the Mollusca, 

 and the Molluscoida, points to a racial form having this structure. 

 This brings us to the difficult and much discussed question of the 

 origin of the Mollusca.* 



Of the theories as to the origin of the Mollusca, two seem to us 

 to demand our special attention, these being the theory that the 

 Mollusca are derived from Turbellaria-like forms and that which 

 derives them from a Trochophore-like ancestor. 



The derivation of the Mollusca from Turbellaria-like forms has much 

 in its favour, especially as it affords a partial explanation of the 

 perplexing conditions of the nervous system. The pedal strands, 

 according to this view, would correspond to the ventral longitudinal 

 nerves of the Turbellaria, while the pleuro-visceral strands correspond 

 to the lateral nerves. The resemblance is specially striking in the ner- 

 vous system of the Amphineura (forms which somewhat resemble the 

 Turbellaria in shape) which consists of four longitudinal strands with 

 connecting commissures. A similar ladder-like nervous system with 

 a ventral and a lateral strand on each side occurs in the Turbellaria 

 (Triclada and especially Gunda). The anus, which is wanting in the 

 Turbellaria, was acquired later, and so was the blood vascular system. 

 The coelom is to be explained by the dilation of the genital glands, 

 for the genital products originate, as was shown, from its epithelial 

 wall. The shell, an important component part of the Molluscan 

 organisation, arose for the protection of the body in the form of 

 a cuticular secretion of the dorsal surface in which were deposited 

 calcareous concretions. The foot, an equally essential part of the 

 organism, resulted from the transformation of the ventral surface of 

 the body, which was used for creeping, into a muscular sole, or else 

 is assumed to be a modification of the ventral sucker.f 



* A great deal has been written on the relationships of the Mollusca. We 

 refrain from discussing the different and often opposite views which have been 

 propounded on this subject as they would merely add length to our account 

 and make it the less clear. We shall only allude in passing to the Trochophore 

 theory adopted by RAY LANKESTER and still more ardently by HATSCHEK, and 

 to LANG'S theory of t.he derivation from Turbellaria-like forms. A list of the 

 most important works on the subject will be found at the end of this chapter. 

 LANG'S view has recently been published in his Text-book of Comparative 

 Anatomy (Engl. Trans., Vol. ii.). 



t THIELE, like LANG, derives the Mollusca from Turbellaria-like forms and 

 regards the ventral sucker of the Polyclada as the organ from which, without 



