308 F. F. LAIDLAW. 



Prosthiostonium). In speaking of the remarkable Polypostia Bergendal [1] remarks that ' tentacle 

 eyes' can scarcely be distinguished. In passing we may notice that like Anonymus, Polypostia 

 is provided with a number of male copulatory organs surrounding the female aperture. The 

 fact is that some of the hinder pairs are not connected with the sperm-ducts. This is regarded 

 by Lang as an indication that the penis in Polyclads is derived from glandular organs, which 

 have only a secondary connection with the sperm-ducts. Whilst this is very probably true, 

 it is possible that in Polypostia we have a stage in the disappearance of the multiple radially 

 arranged penes that appear to be an ancestral character. In this connection reference may 

 be made to the paired penes of certain Pseudoceridae. 



Origin of Nuchal Tentacles. Of the origin of the tentacles of the Acotylea two views 

 are held : — 



(1) That adopted by Lang, who, following Chun, brings forward evidence for supposing 



the Polycladida to be derived from a Ctenophore-like ancestor; and homologizes 

 the dorsal tentacles of the Planoceroids with the retractile tentacles of certain 

 Ctenophores. 



(2) That of Willey [17], who also derives the Polyclads from a radially symmetrical 



Ctenophore-like ancestor. He suggests that the dorsal tentacles of Polyclads 

 are homologous with the sensory tentacles found in certain Ctenophores, and 

 especially in Ctenoplana. 



I here have to put forward a third suggestion, which differs completely from either of the 

 preceding. Without questioning the probability of the origin of the Polyclads from a radial 

 ancestral form, or the possibility of that form being also allied to a form ancestral to the 

 Ctenophores, and avoiding any discussion as to the axial relationship of two groups, I 

 consider that the tentacles of the Polyclads, whether marginal or nuchal, 



(1) are a structure peculiar to the group; 



(2) that they originated as marginal organs in connection with the eye-spots, some- 



what in the way I have suggested above ; 



(3) that in the Cotylea they still retain this condition ; 



(4) that in the bulk of the Acotylea they have shifted back from the margin and 



come to lie dorsally. 



This theory offers a more satisfactory explanation of the relationship of the Acotylea to 

 the Cotylea than is possible on either of the two first. For, if we adopt either of these, 

 we must suppose that the Cotylea are descended from forms which have lost all traces of 

 the nuchal tentacles, and have in most cases acquired marginal tentacles. Now the Lepto- 

 planidae have lost the dorsal or nuchal tentacles but have in almost every species retained 

 evidence of their existence in the tentacle eye groups, whilst none of them have developed 

 marginal tentacles, although some of them, e.g. Discocelis, have retained marginal eyes. On 

 the other hand no Cotylean species ever exhibits any trace of a nuchal tentacle eye group. 



On my theory there is no reason to regard the Cotylea as less primitive than the 

 Acotylea; in fact the balance of evidence distinctly inclines to the opposite view. If we 

 leave out of account the elongated, obviously highly specialized forms in either group, i.e. the 

 Prosthiostomidae and Cestoplanidae, the Cotylea present far greater uniformity and less com- 

 plexity of organization of the genital apparatus than we find in the Acotylea ; whilst most 



