80 PROCEEDINGS OF THE MALACOLOGICAL SOCIETY. 



and Mollusca on the other. Conklin (6, pp. 192 et seqq.) dwells 

 on the remarkable similarity between the early pretrochophore 

 stages of development in the Annelida and Mollusca. Korschelt 

 and Heider {25, pp. 321-2 and 327) incline to the Annelid affinities 

 of the Mollusca ; they point to the trochophore stage (trochosphere of 

 Lankester) in both as indicating a common ancestry, and proceed to 

 construct a primitive mollusc therefrom. This is always a dangerous 

 proceeding, and, as they seem to have expected themselves, subsequent 

 discoveries have demolished much of their structure. Other points 

 of agreement that they indicate as existing between the two phyla 

 are — "the conditions of the coelom," which "agree in such a striking 

 manner , . . that it is difficult to believe that two structures so 

 remarkably alike arose in different ways" {25, p. 326), the occurrence 

 of the nephridia and their connection with the coelom, and the great 

 resemblance of the circulatory system of the Mollusca to that of the 

 Annelida (p. 327). 



Pelseneer is yet more explicit, and as early as 1892 {36, pp. 371-372) 

 concluded that the Mollusca came nearest to the Polychseta Errantia, 

 and more especially to Eunice. The resemblances on which he relies are 

 set forth more fully in his "Mollusques Archa'iques" {39, pp. 85-87); 

 they include the similarity in the early development in the two classes 

 that has already been alluded to, the structure of the eyes and organs 

 of taste, of the spicules and the generative organs, while in addition 

 the Eunicidae possess a muscular pharynx, with a c^cal invagination 

 under the oesophagus enclosing chitinous denticles, all which points 

 are paralleled in the buccal mass, radula sac, and radula of the 

 Mollusca. In Staurocephalus ^ the similarity of the chitinous denticles 

 to radula teeth is striking. Moreover, the anterior portion of the 

 nervous system in Eunice and the Mollusca is, as Pelseneer shows, 

 wonderfully similar. He also would, therefore, derive the Annelida 

 and the Mollusca from a common stock. 



It may further be noted that, although the modern morphologist is, 

 as a rule, as chary of mentioning it as the shell- collector is of alluding 

 to the 'nasty animal,' not a few molluscs do possess shells, and that 

 while the morphologist is apt to treat its occurrence as a matter of no 

 moment and a phenomenon that might readily arise at any time. 

 Nature does not seem to acquiesce in this opinion. For except in the 

 case of some of the most highly specialized, shell-less, forms of all, in 

 which by acceleration of development the stage has been suppressed, 

 all molluscs alike start in the egg with an exogastric nautiloid shell. 

 The shell, therefore, being an embryonic character, would point to an 

 ancestry that possessed this feature, and it is suggestive that of all the 

 Worms the Polychseta alone furnish examples provided with calcareous 

 tests {Spirorlis, Serpula), although in their case there is no organic 

 connection between the animal and its shelly tube. 



1 "What the valid name of this "Worm may be is at present uncertain. Stanrocejjhalus 

 is preoccupied for Trilobita, and the synonyms, Anisoceras and Prw)iognathHs, 

 quoted by Carus, are equally forestalled for other branches of the Animal 

 Kingdom. 



