330 GENERAL CONSIDERATIONS ON THE MOLLUSCA. 



supposed relation to the Lamellibranchia and the Cephalopoda being 

 untenable.* The Solenoconchae are an aberrant although insignificant 

 branch of the Molluscan stock, certain resemblances between them 

 ^a-nd the Lamellibranchia are to be explained by the fact that they 

 both branched off from the same primitive form. 



The Lamellibranchia also are much specialised, but may still, 

 through their lowest representatives, be related to the primitive form. 

 The Protobranchia, for instance, still possess a foot with a creeping 

 ;sole, as well as bipectinate gills. The foot and the gills in the 

 higher forms, however, though modified, may still be traced back to 

 the fundamental type. The creeping sole is, in any case, lost in 

 consequence of their burrowing habit, but, 011 the other hand, one of 

 the pedal glands which are found in the different divisions of the 

 Mollusca develops into the byssal apparatus. 



The reduction of the head and absence of the radula, structures 

 which are such constant features in nearly all other Molluscs, are 

 characteristic of the Lamellibranchia. It has been said, no doubt 

 rightly, that the radula has been lost ; it is occasionally also wanting 

 in other forms whose relations possess it as, for instance, in various 

 Opisthobranchia (Phyllidia> Doridt'um, Doridopsis, Tethys, etc.).-f 



The shell of the Lamellibranchia has a specially typical develop 

 ment. At first it is shaped like a shallow bowl, lying upon the back, 

 like the shell assumed to have been possessed by the primitive form ; 

 later, however, it bends over on the two sides, calcifies as two pieces, 

 and thus assumes the typical bivalve form. The formation of the 

 mantle corresponds to that of the shell. 



Among the internal organs, the nervous system, the circulatory 

 apparatus, the pericardium (coelom) and the nephridia form in the 

 way usual among the Mollusca, and this probably also may be said of 

 these organs in the Cephalopoda. 



The Cephalopoda. Of the external organs of this class, the mantle 

 .and gills also resemble those of other Molluscs. The shell too may 

 be derived from a simpler form, as is evident from the rounded 

 chambers found in the embryo. The highly complicated form of the 



* PLATE, in his recent work on the anatomy of Dcntalium, gives a detailed 

 -account of the relationships of these forms (see Literature to Chapter 

 XXXI., No. 3, p. 98). 



t According to SIMROTH (No. 17), Tethys (and the related form Melibe), for 

 instance, does not require the radula because its food is soft. A Prosobranch 

 also (Magilus) has no radula. It lives in a tube covered by corals and feeds 

 -on the offal of these animals. [The Prosobranch families Pyramidellidae and 

 Eulimidae are also devoid of radulae. ED]. 



