10 ßr. Asajiro Oka : 



wall of the head may be most clearly understood by examining 

 the transverse and longitudinal sections of this region. In PI. I., 

 fig. 3, representing a part of cross section of the head, the test is 

 found to comprise the following parts: 1) a superficial layer cover- 

 ing the outer surface of the head, 2) thin partitions separating the 

 thoracic regions of various zooids, and 3) more or less thickened 

 masses in which their abdominal regions are imbedded. The 

 superficial layer, which is nearly of the same thickness through- 

 out, not only covers the external surface of the anterior ends of 

 the zooids, but is turned inwards at the branchial aperture and 

 reaches the bases of the tentacles so as to form an internal lining 

 of the branchial siphon. The walls of test substance forming 

 partitions between the thoracic regions of the zooids are in some 

 places much thicker than the superficial layer, but in other places 

 they are exceedingly thin. Their thickness depends on the 

 configuration of the zooids the interstices between which they fill 

 up, but on the whole the test is not particularly well developed 

 in these parts. It is in the inner half of the longitudinal ridges 

 that the common test attains its highest development. Here it is 

 found to form masses of more or less considerable thick- 

 ness in which the abdominal parts of the zooids are completely 

 imbedded. The masses also contain blood vessels to be described 

 further on. The extent of the common investing mass in the 

 longitudinal ridges is perhaps better shown in PI. I., fig. 4, which 

 represents a part of longitudinal section through the side wall of 

 the head. From this figure it will be seen that the posteror 

 halves of the abdomens are imbedded in a continuous mass of 

 test substance running longitudinally, Avhile the anterior halves 

 containing the oesophagus and stomach are covered separately by 

 a thin layer of test. Where the peribranchial wall is wanting, 

 as shown in the figure, the branchial sacs as well as the 

 anterior portions of abdomens are separated from one another 

 simply by the common peribranchial space (p'"&.) directly 

 continuous with the adjacent longitudinal furrow. The trans- 

 verse walls of test intervening between the anterior regions of the 

 branchial sacs are as a rule of the same thickness as the super- 



