290 DR. ARTHUR WILLET ON 



Zool. Anz. xxviii. 1904, pp. 273-322), disposing of excellent data, 

 maintained that the change from a radial to a bilateral form is 

 best understood by the transition from the pelagic to the 

 benthonic habit, on the assumption that Bilateralia are to be 

 derived from pelagic, radiate, marine animals. He reconstructs, 

 in imagination, the ancestral type of Annelids, as a pelagic, 

 spherical, octoradiate, bipolar Coelenterate, to which he applies 

 the phylogenetic designation : Octoradiata-Bipolaria. His idea 

 of the divergent descent of Otenophora and Trochophora may be 

 expressed in graphic form : — 



Bipolaria. 



Trochophora, Ctenophora. 



Woltereck refers quite simply to Cteno2olana as a '' creeping 

 Ctenophore." As a creeping Ctenophore its most conspicuous 

 distinctions are a permanent dorsiventrality, the habit of 

 reptation, the presence of pairs of organs, and the possession 

 of a circular mouth. These four qualities are shared by the 

 Polyclade Turbellarians, with bilaterality in addition. Never- 

 theless, in comparison with the flatworms, these features of 

 Gtenoplana are the result of convergence ; it is not in the least 

 degi'ee related to the flatworms because of its flatworm habit. 

 To those who have not seen Q^enoplana this may have trifling 

 significance ; but after seeing it in the living animal, it becomes 

 fraught with meaning, bearing upon the principles and efiects of 

 adaptation. 



Actinotrocha, the larva of Phoronis, appeared in the tow, a 

 solitary example, on August 19th, from about five fathoms. The 

 typical long tentacles were marked with brown pigment ; and 

 behind their bases were the small replacing tentacles, indicating 

 that the metamoiphosis was at hand. On the preoral lobe or 

 hood, there was a projecting ectodermal knob, a pa,pilla, ciliated 

 and presunyibly sensory, a little ventral to the apical plate. Such 

 an ectodermal knob on the hood is figured by Marc de Selys- 

 Longchamps in Actinotrocha hrovmei fi-om Plymouth, England, 

 the larva of an undiscovered Phoronis. This larva bears a striking 

 but resemblance to the one at St. Andrews, and I have no doubt but 

 that it is the same species. [M. de Selys-Longchamps, ^^ Phoronis," 

 Fauna u. Flora des Golf es von Neapel, 30th Monograph. Berlin, 

 1907 ; see p. 190 and pi. xi. fig. 31 .] 



In front of the anal peritroch, on the ventral side, was an 

 oblique depression, marking the site of the invagination of the 

 metasome into the interior of the larva. 



The eversion of the Phoronis-hody or metasome took place 

 about 3 P.M. in the watchglass. As I had not expected such a 



