42 
SUMMARY OF CURRENT RESEARCHES RELATING TO 
Incertse Sedis. 
Structure of Actinotrocha.* — Mr. A. T. Masterman has a memoir on 
the structure of Actinotrocha , considered in relation to the suggested 
chordate affinities of Phoronis. He points out the close harmony of 
structure which is to he found between the larval forms of Actinotrocha 
and Tornaria. The chief of these are the following : — A much overhanging 
preoral lobe and, in some cases, two eye-spots ; a preoral ciliated ring ; 
a postoral ring ; a circumanal ring of cilia ; mesoderm forming five 
coelomic pouches, one preoral and two pairs postoral. Mr. Masterman 
thinks that the study of the structure of Phoronis and of its larval form 
Actinotrocha entirely points to the close alliance of the genus to the 
members of the group of the Hemichordata, and Balanoglossus in parti- 
cular. The present structure of Phoronis points to a marked degenera- 
tion, due to a sedentary life, for the highest stage of the organs is 
reached in Actinotrocha just before the metamorphosis, which results in 
the transformation to the adult condition. In the next place, the struc- 
ture [of Actinotrocha conforms to the hemichordate type extremely 
closely, even to minute particulars. As a general result of his work, 
the author proposes a further arrangement of the Chordata, forming the 
grouj) Diplochorda for Phoronis. He appends the following view : — 
Chordata. A. Trimetamera. 
1. Diplochorda. 
Phoronis. 
2. Hemichorda. 
Balanoglossus , 
Cephalodiscus } 
% Bhabdopleura. 
B. Polymetamera. 
3. Urochorda. 
4. Cephalochorda. 
5. Holochorda. 
Echinoderma. 
Function of Polian Vesicles in Sea-TJrchin.j — Herr J. vonUexkiill 
uses the term “ Polian vesicles ” in the sense in which Delle Chiaje used 
it, in reference to the five interradial bladders ( Zahnblasen ) which lie 
above the lantern. The history of the term and of what has been said 
and figured in regard to the organs in question is sketched in an instruc- 
tive, if somewhat humiliating, fashion. The only figure which the 
author finds to commend is that given by Romanes and Ewart. The 
radial bladders ( Gabelblasen ) between the interradials ( Zahnblasen ) 
have suffered similar maltreatment. Both Gabelblasen and Zahnblasen 
are diverticula of the membrana limitans lanternae. Uexkiiirs investi- 
gation concerns their function. After discussing the conditions of 
pressure within Sphser echinus, he shows that the bladders are not to be 
interpreted as otolithic, nor as locomotor. The “ compass ” muscuHture 
serves solely to regulate the pressure within the bladder system of the 
membrana limitans ; the regulated changes of pressure subserve respira- 
tion and the masticating movements. Increase of pressure within the 
vesicular space serves to protrude the oesophageal papillae ; the changes 
* Proc. Roy. Soc. Edinburgh, xxi. (1896) pp. 129-37 (4 figs.). 
f MT. Zool. Stat. Neapel, xii. (1896) pp. 463-76 (1 pi.). 
